before, RelWithDebInfo linking gave this warning:

    src/mpack/conv.h:36:16: warning: type of ‘mpack_unpack_boolean’ does not match original declaration [-Wlto-type-mismatch]
       36 | MPACK_API bool mpack_unpack_boolean(mpack_token_t t) FUNUSED FPURE;
          |                ^
    src/mpack/conv.c:196:16: note: return value type mismatch
      196 | MPACK_API bool mpack_unpack_boolean(mpack_token_t t)
          |                ^

Currently this is more or less a straight off reimplementation,
but this allow further optimizations down the line, especially
for avoiding memory allocations of rpc objects.

Current score for "make functionaltest; make oldtest" on a -DEXITFREE build:

is 117 055 352 xfree(ptr != NULL) calls (that's NUMBERWANG!).

Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>
Co-authored-by: Brede Yabo Sherling Kristensen <bredeyabo@hotmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: István Donkó <istvan.donko@gmail.com>
Co-authored-by: Julian Berman <Julian@GrayVines.com>
Co-authored-by: bryant <bryant@users.noreply.github.com>
Co-authored-by: Michael Lingelbach <m.j.lbach@gmail.com>
Co-authored-by: nlueb <9465658+nlueb@users.noreply.github.com>
Co-authored-by: Leonhard Saam <leonhard.saam@yahoo.com>
Co-authored-by: Jesse Wertheim <jaawerth@gmail.com>
Co-authored-by: dm1try <me@dmitry.it>
Co-authored-by: Jakub Łuczyński <doubleloop@o2.pl>
Co-authored-by: Louis Lebrault <louis.lebrault@gmail.com>

Problem:
1. "unpack" has an unrelated meaning in Lua:
   https://www.lua.org/manual/5.1/manual.html#pdf-unpack
2. We already have msgpackparse()/msgpackdump() and
   json_encode()/json_decode(), so introducing another name for the same
   thing is entropy.

Solution:
- Rename vim.mpack.pack/unpack => vim.mpack.encode/decode

Caveat:
This is incongruent with the `Unpacker` and `Packer` functions.
- It's probably too invasive to rename those.
- They also aren't part of our documented interface.
- This commit is "reversible" in the sense that we can always revert
  it and add `vim.mpack.encode/decode` as _aliases_ to
  `vim.mpack.pack/unpack`, at any time in the future, if we want
  stricter fidelity with upstream libmpack. Meanwhile,
  `vim.mpack.encode/decode` is currently the total _documented_
  interface of `vim.mpack`, so this change serves the purpose of
  consistent naming in the Nvim stdlib.