Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.
- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
"know" about the various magic symbols.
So they harm DX, which means they serve no purpose at all.
Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
`t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
`utf_char2cells()` calls `utf_printable()` twice (sometimes indirectly,
through `vim_isprintc()`) for characters >= 128. The function can be
refactored to call to it only once.
`utf_printable()` uses binary search on ranges of unprintable characters
to determine if a given character is printable. Since there are only 9
ranges, and the first range contains only one character, binary search
can be replaced with SSE2 SIMD comparisons that check 8 ranges at a
time, and the first range is checked separately. SSE2 is enabled by
default in GCC, Clang and MSVC for x86-64.
Add 3-byte utf-8 to screenpos_spec benchmarks.
