* Introducing move semantics vor pushKV and push_back. In many use cases
  the const& calls unnecessarily create a copy of the argument, when
  objects can be moved this does not happen.

* Reduce creation of temporary strings. `json_escape` now appends to
  an existing string instead of creating a temporary, and UniValue::write
  now uses an UniValue::append where possible.

* Use `std::to_string` instead of `std::ostringstream` for `setNumStr`

In a benchmark with Bitcoin's BlockToJsonVerbose, using the move methods
where possible speeds up JSON generation by about a factor of 2.

Prevent unnecessary copies of UniValue objects by using `std::move`
where possible.

This speeds up the `BlockToJsonVerbose` benchmark from 66.9 ms/op to
36.6 ms/op on an intel i7 locked to 3.2GHz.

|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | `BlockToJsonVerbose`
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|       66,877,763.00 |               14.95 |    0.1% |  549,960,313.00 |  211,619,093.00 |  2.599 | 101,638,376.00 |    0.4% |      0.75 | `master`
|       57,915,056.00 |               17.27 |    1.2% |  464,866,030.00 |  183,491,588.00 |  2.533 |  85,882,830.00 |    0.5% |      0.65 | `univalue: move semantics, reduce temporary strings`
|       36,585,334.00 |               27.33 |    1.6% |  310,260,492.00 |  116,584,209.00 |  2.661 |  50,966,277.00 |    0.4% |      0.41 | `make use of univalue's new move semantic`

Note that there are still places where `std::move` could be introduced.

std::string's push_back can be very time consuming, even with
reserve(). If possible, this should be avoided. In the case of `HexStr`
this is easily possible, and since it is heavily used, the advantage for
`BlockToJsonVerbose` are significant:

|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | `BlockToJsonVerbose`
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:----------
|       36,585,334.00 |               27.33 |    1.6% |  310,260,492.00 |  116,584,209.00 |  2.661 |  50,966,277.00 |    0.4% |      0.41 | `make use of univalue's move semantic`
|       28,508,105.00 |               35.08 |    0.2% |  244,933,429.00 |   90,874,112.00 |  2.695 |  40,138,962.00 |    0.4% |      0.32 | `optimize HexStr`