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.
