Adding fuzzers for unsafe code.

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This adds a `bytes` submodule for operating on byte strings that might
contain invalid UTF-8. Where possible I have switched the functions that
operate on `str` to use the `bytes` functions internally to avoid
duplicating code and eliminate the potential for differing behavior
between the two functions.

It includes trivial tests that confirm that the `bytes` version of
functions actually work on invalid UTF-8.

Fixes #12

Add support for operating on byte strings

Ref: GHSA-r7qv-8r2h-pg27

Ref: GHSA-r7qv-8r2h-pg27

- Deprecate quote APIs in favor of `try_` equivalents that complain
  about nul bytes.

- Also add a builder API, which allows re-enabling nul bytes without
  using the deprecated interface, and in the future can allow other
  things (as discussed in quoting_warning).

- Add documentation about various security risks that remain,
  particularly with interactive shells.

- Add fuzzers that actually verify round-trippability of the quote APIs
  against various shells, Python `shlex`, and C `wordexp`.

  - These are separate crates (as opposed to just being different files
    under `fuzz/fuzz_targets`) because they have different dependencies
    and build steps, and I don't want to agglomerate them all together.
    I've put them in the same workspace at least.

  - Also, check in Cargo.lock for the fuzzers, since they are binaries.

- Add explicit MSRV of 1.46.0.

  This crate didn't previously have an explicit MSRV, but `cargo msrv`
  tells me that shlex 1.2.0 works down to Rust 1.36.0.

  Since this is a security fix, ideally the MSRV wouldn't be bumped at
  all, but that's not really feasible since the new API uses
  `#[non_exhaustive]`, which was unstable in Rust 1.36.0.  In case anyone
  is stuck on old Rust versions, I separately released a shlex 1.2.1 that
  only has the fix for `{`/`}`/`\xa0`, without the API changes.

  However, even for the full release I'd still like to keep the MSRV
  reasonably old.  I picked 1.46.0 because it's the first version that
  wouldn't require completely redoing the `const fn` bitmask.

- Add more authors to Cargo.toml based on Git commits.