In case nvim A sends nvim_error_event to nvim B, it would
respond with another nvim_error_event due to unknown
request name. Fix this by adding dummy request handler for now.

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
https://github.com/vim/vim/commit/ff06f283e3e4b3ec43012dd3b83f8454c98f6639

Fix a mistake in the porting of patch 8.1.0098.
Cherry-pick Test_run_excmd_with_text_locked() from patch 8.2.0270.
Cherry-pick test_gf.vim changes from patch 8.2.0369.
Cherry-pick message change from later patches.

Problem:
1. The main log routine does not protect itself against recursion.
   log_lock() doesn't guard against recursion, it would deadlock...
2. 22b52dd462e5 (#11501) regressed 6f27f5ef91b3 (#10172), because
   set_init_1..process_spawn tries to log (see backtrace below), but the
   mutex isn't initialized yet. Even if the mutex were valid, we don't
   want early logging to fallback to stderr because that can break
   embedders when stdio is used for RPC.

    frame 1: 0x00000001001d54f4 nvim`open_log_file at log.c:205:7
    frame 2: 0x00000001001d5390 nvim`logmsg(log_level=1, context="UI: ", func_name=0x0000000000000000, line_num=-1, eol=true, fmt="win_viewport") at log.c:150:20
    frame : 0x000000010039aea2 nvim`ui_call_win_viewport(grid=2, win=1000, topline=0, botline=1, curline=0, curcol=0, line_count=1) at ui_events_call.generated.h:321:3
    frame 4: 0x00000001003dfefc nvim`ui_ext_win_viewport(wp=0x0000000101816400) at window.c:939:5
    frame 5: 0x00000001003ec5b4 nvim`win_ui_flush at window.c:7303:7
    frame 6: 0x00000001003a04c0 nvim`ui_flush at ui.c:508:3
    frame 7: 0x00000001002966ba nvim`do_os_system(argv=0x0000600000c0c000, input=0x0000000000000000, len=0, output=0x0000000000000000, nread=0x00007ff7bfefe830, silent=false, forward_output=false) at shell.c:894:3
    frame 8: 0x0000000100295f68 nvim`os_call_shell(cmd="unset nonomatch; vimglob() { while [ $# -ge 1 ]; do echo \"$1\"; shift; done }; vimglob >/var/folders/gk/3tttv_md06987tlwpyp62jrw0000gn/T/nvimwwvwfD/0 ~foo", opts=kShellOptExpand | kShellOptSilent | kShellOptHideMess, extra_args=0x0000000000000000) at shell.c:663:18
    frame 9: 0x0000000100295845 nvim`call_shell(cmd="unset nonomatch; vimglob() { while [ $# -ge 1 ]; do echo \"$1\"; shift; done }; vimglob >/var/folders/gk/3tttv_md06987tlwpyp62jrw0000gn/T/nvimwwvwfD/0 ~foo", opts=kShellOptExpand | kShellOptSilent | kShellOptHideMess, extra_shell_arg=0x0000000000000000) at shell.c:712:14
    frame 10: 0x0000000100294c6f nvim`os_expand_wildcards(num_pat=1, pat=0x00007ff7bfefeb20, num_file=0x00007ff7bfefee58, file=0x00007ff7bfefee60, flags=43) at shell.c:328:7
    ...
    frame 23: 0x000000010028ccef nvim`expand_env_esc(srcp=",~foo", dst="~foo", dstlen=4094, esc=false, one=false, prefix=0x0000000000000000) at env.c:673:17
    frame 24: 0x000000010026fdd5 nvim`option_expand(opt_idx=29, val=",~foo") at option.c:1950:3
    frame 25: 0x000000010026f129 nvim`set_init_1(clean_arg=false) at option.c:558:19
    frame 26: 0x00000001001ea25e nvim`early_init(paramp=0x00007ff7bfeff5f0) at main.c:198:3
    frame 27: 0x00000001001ea6bf nvim`main(argc=1, argv=0x00007ff7bfeff848) at main.c:255:3

Solution:
1. Check for recursion, show "internal error" message.
    - FUTURE: when "remote TUI" is merged, can we remove log_lock()?
2. Skip logging if log_init wasn't called yet.

This avoids generating khash tables at runtime, and is consistent with
how evalfuncs lookup work.

Add support for adding and removing custom user commands with the Nvim
API.

Fixes #15147 and fixes #15497. Also sketch "subdir" caching. Currently
this only caches whether an rtp entry has a "lua/" subdir but we could
consider cache other subdirs potentially or even "lua/mybigplugin/"
possibly.

Note: the async_leftpad test doesn't actually fail on master, at least
not deterministically (even when disabling the fast_breakcheck
throttling). It's still useful as a regression test for further changes
and included as such.

Do not copy a lot of lua strings (dict keys) to just strequal() them
Just compare them directly to a dedicated hash function.

feat(generators): HASHY McHASHFACE

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")

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.