Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

Note that it is not possible for msgpack_unpack_next() and
msgpack_unpacker_next() to return MSGPACK_UNPACK_EXTRA_BYTES, so it
should be fine to abort() on that.

Lua 5.1 doesn't support string hex escapes (\xXX) like VimL does (though
LuaJIT does), so convert them to decimal escapes (\DDD) in tests.

Strings that previously decoded into a msgpack special for representing
BINs with NULs now convert to Blobs. It shouldn't be possible to decode
into this special anymore after this change?

Notably, Lua strings with NULs now convert to Blobs when passed to VimL.

In order to generate INT64_MIN from literal values, it's necessary to
use "-0x7fffffffffffffff - 1".  Using "-0x8000000000000000" causes the
value to get clamped to INT64_MAX and then negated.