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`.
Problem:
- Lua<=>API roundtrips
- Although we prefer Lua for most business-logic code, doing this
conversion in C makes sense in this case because:
1. setting options is a hot path
2. most of the options logic lives in C
3. the current arrangement is MORE verbose and requires MORE code
Solution:
Move conversion to a C util.
- nvim_set_option_value passes the raw Object (scalar, Array, or Dict)
to `object_as_optval_for()` which flattens it to the canonical `:set`
string and validates the type.
- drop `convert_value_to_vim`, eliminate its roundtrip.
