Analogous to nodejs's `on('data', …)` interface, here on_key is the "add
listener" interface.

ref 3ccdbc570d85 #12536

BREAKING_CHANGE: vim.register_keystroke_callback() is now an error.

fixes #15524

Note: this is obviously a quickfix. A scalabe solution will
involve being able to specify a _list_ of modules to be
put into packages.preload, without needing to manually copypasta
a blurb of C code. Perhaps even involving bytecode for
static builds (to speedup initialization)

Port VimL's method call syntax - vim-patch:8.1.{1638,1800,1803,1807,1809,1816,1820,1821,1828,1834,1835,1861,1863,1878,1879,1888,1909,1911,1912}

Note: the reason for removing them is not that there after this refactor
is no use of them, but rather that having them available is an
anti-pattern: they manange an _extra_ heap allocation which has
nothing to do with the functionality of the map itself (khash
manages the real buffers internally). In case there happens to
be a reason to allocate the map structure itself later, this
should be made explicit using xcalloc/xfree calls.

perf(map): get rid of unnecessary pointer indirections for maps.

* feat(api): add lua C bindings for xdiff

* chore: opt.hunk_lines -> opt.result_type

opt.on_hunk now takes precedence over opt.result_type

* chore: fix indents

Fix indents

* chore: change how priv is managed

Assign priv NULL and unconditionally apply XFREE_CLEAR to it when
finished.