00e9b97 refactor: Move fs.* to util/fs.* (TheCharlatan)
106b46d Add missing fs.h includes (TheCharlatan)
b202b3d Add missing cstddef include in assumptions.h (TheCharlatan)
18fb363 refactor: Extract util/fs_helpers from util/system (Ben Woosley)

Pull request description:

  This pull request is part of the `libbitcoinkernel` project bitcoin/bitcoin#24303 https://github.com/bitcoin/bitcoin/projects/18 and more specifically its "Step 2: Decouple most non-consensus code from libbitcoinkernel". This commit was originally authored by empact and is taken from its parent PR #25152.

  #### Context

  There is an ongoing effort to decouple the `ArgsManager` used for command line parsing user-provided arguments from the libbitcoinkernel library (bitcoin/bitcoin#25290, bitcoin/bitcoin#25487, bitcoin/bitcoin#25527, bitcoin/bitcoin#25862, bitcoin/bitcoin#26177, and bitcoin/bitcoin#27125). The `ArgsManager` is defined in `system.h`. A similar pull request extracting functionality from `system.h` has been merged in bitcoin/bitcoin#27238.

  #### Changes

  Next to providing better code organization, this PR removes some reliance of the tree of libbitcoinkernel header includes on `system.h` (and thus the `ArgsManager` definition) by moving filesystem related functions out of the `system.*` files.

  There is already a pair of `fs.h` / `fs.cpp` in the top-level `src/` directory. They were not combined with the files introduced here, to keep the patch cleaner and more importantly because they are often included without the utility functions. The new files are therefore named `fs_helpers` and the existing `fs` files are moved into the util directory.

  Further commits splitting more functionality out of `system.h` are still in #25152 and will be submitted in separate PRs once this PR has been processed.

ACKs for top commit:
  hebasto:
    ACK 00e9b97

Tree-SHA512: 31422f148d14ba3c843b99b1550a6fd77c77f350905ca324f93d4f97b652246bc58fa9696c64d1201979cf88733e40be02d262739bb7d417cf22bf506fdb7666

…ibrary, interface_ui from validation.

7d3b350 refactor: Move system from util to common library (TheCharlatan)
7eee356 refactor: Split util::AnyPtr into its own file (TheCharlatan)
44de325 refactor: Split util::insert into its own file (TheCharlatan)
9ec5da3 refactor: Move ScheduleBatchPriority to its own file (TheCharlatan)
f871c69 kernel: Add warning method to notifications (TheCharlatan)
4452707 kernel: Add progress method to notifications (TheCharlatan)
84d7145 kernel: Add headerTip method to notifications (TheCharlatan)
447761c kernel: Add notification interface (TheCharlatan)

Pull request description:

  This pull request is part of the `libbitcoinkernel` project bitcoin/bitcoin#27587 https://github.com/bitcoin/bitcoin/projects/18 and more specifically its "Step 2: Decouple most non-consensus code from libbitcoinkernel".

  ---

  It removes the kernel library's dependency on `util/system` and `interface_ui`. `util/system` contains networking and shell-related code that should not be part of the kernel library. The following pull requests prepared `util/system` for this final step: bitcoin/bitcoin#27419 bitcoin/bitcoin#27254 bitcoin/bitcoin#27238.

  `interface_ui` defines functions for a more general node interface and has a dependency on `boost/signals2`. After applying the patches from this pull request, the kernel's reliance on boost is down to `boost::multiindex`.

  The approach implemented here introduces some indirection, which makes the code a bit harder to read. Any suggestions for improving or reworking this pull request to make it more concise, or even reworking it into a more proper interface, are appreciated.

ACKs for top commit:
  MarcoFalke:
    re-ACK 7d3b350 (no change) 🎋
  stickies-v:
    Code Review ACK 7d3b350
  hebasto:
    re-ACK 7d3b350, only last two commits dropped since my [recent](bitcoin/bitcoin#27636 (review)) review.

Tree-SHA512: c8cfc698dc9d78e20191c444708f2d957501229abe95e5806106d1126fb9c5fbcee686fb55645658c0107ce71f10646f37a2fdf7fde16bbf22cbf1ac885dd08d

…e time (#5693)

## Issue being fixed or feature implemented
Some headers include other heavy headers, such as `logging.h`,
`tinyformat.h`, `iostream`. These headers are heavy and increase
compilation time on scale of whole project drastically because can be
used in many other headers.

## What was done?
Moved many heavy includes from headers to cpp files to optimize
compilation time.
In some places  added forward declarations if it is reasonable.

As side effect removed 2 circular dependencies:
```
"llmq/debug -> llmq/dkgsessionhandler -> llmq/debug"
"llmq/debug -> llmq/dkgsessionhandler -> llmq/dkgsession -> llmq/debug"
```


## How Has This Been Tested?
Run build 2 times before refactoring and after refactoring: `make clean
&& sleep 10s; time make -j18`

Before refactoring:
```
real    5m37,826s
user    77m12,075s
sys     6m20,547s

real    5m32,626s
user    76m51,143s
sys     6m24,511s
```

After refactoring:
```
real    5m18,509s
user    73m32,133s
sys     6m21,590s

real    5m14,466s
user    73m20,942s
sys     6m17,868s
```

~5% of improvement for compilation time. That's not huge, but that's
worth to get merged

There're several more refactorings TODO but better to do them later by
backports:
 - bitcoin#27636
 - bitcoin#26286
 - bitcoin#27238
 - and maybe this one: bitcoin#28200


## Breaking Changes
N/A

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone