(Neo)Vim uses a jump table based on the first character of the option's
name. This means for it to work all options with the same first
character must be together. If this is violated, some options may seem
to disappear, which can sometimes happen during merges if not careful.

This commit adds a check to the gen_options.lua script so the build will
fail if this constraint is violated.

These functions do not involve msgpack. Initially the nvim api was
sometimes called the "msgpack API", but entry points from vim script
and lua are equally valid (and don't need to reference "msgpack")

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.

Problem:    More functions can be used as a method.
Solution:   Add append(), appendbufline(), assert_equal(), etc.
            Also add the :eval command.
https://github.com/vim/vim/commit/25e42231d3ee27feec2568fa4be2aa2bfba82ae5

:eval is already ported.

Problem:    All builtin functions are global.
Solution:   Add the method call operator ->.  Implemented for a limited number
            of functions.
https://github.com/vim/vim/commit/ac92e25a33c37ec5becbfffeccda136c73b761ac

- Note that to *exactly* port hunk @@ -7376,18 +7444,19 from
  handle_subscript(), we need the :scriptversion patches (I have an open
  PR for those, but this patch works fine without them anyway).
- Port call_internal_func() from v7.4.2058.
- Adjust some error messages in tests, as they rely on the Blob patches.
- Add a modeline to test_method.vim.

Ignore the global_functions and base_method tables and prefer the
current GPerf implementation. Instead, add an extra base_arg field to
VimLFuncDef that holds the number of the argument to use as the base
(1-indexed, so that 0 may be used to refer to functions that cannot be
used as methods).

This also means we support using any argument as a base from the get-go,
rather than just the first (Vim includes this ability in future patches,
however).

To mark a function as usable as a method, use the "base" key as
described in eval.lua.

It might come as a schocking surprise, but the defaults we support
are the NEOVIM defaults.