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:
Listing individual "skipped" notices for each non-actionable or
NVIM_TEST_INTEG test is noisy and makes the "skipped" list overwhelming
and less meaningful.

Solution:
- Mark non-actionable tests with "N/A".
- Report NVIM_TEST_INTEG tests as a separate 1-line summary.

Problem:
On Windows, path separators may become inconsistent for various reasons,
which makes normalization quite painful.

Solution:
Normalize paths to `/` at the entry boundaries and always use it
internally, converting back only in rare cases where `\` is really
needed (e.g. cmd.exe/bat scripts?).

This is the first commit in a series of incremental steps.

Note:
* some funcs won't respect shellslash. e.g. `expand/fnamemodify`
* some funcs still respect shellslash, but will be updated in a follow
  PR. e.g. `ex_pwd/f_chdir/f_getcwd`
* uv's built-in funcs always return `\`. e.g. `uv.cwd/uv.exepath`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Problem:
- Builtin "Vimscript" functions (f_xx) are mostly implemented in C.
  Partly that's because there is some boilerplate required to call out
  to Lua.
- Calls to `vim.fn.foo()` always marshall over the Lua <=> Vimscript
  ("typval") bridge, even if `fn.foo()` is implemented entirely in Lua:
  ```
  Lua => typval => Object => Lua => Object => typval => Lua.
  ```

Solution:
Functions declared in eval.lua with `func_lua` are implemented in
entirely in Lua (`_core/vimfn.lua`).

- `gen_eval.lua` wires `func_lua` entries to `lua_wrapper`, which handles
  the typval conversion for Vimscript callers (slow path).
- `nlua_call()` detects `func_lua` functions and calls the Lua
  implementation directly. This eliminates all conversion overhead for
  Lua callers (fast path).
- Validate at build-time that `func`, `func_float`, and `func_lua` are
  mutually exclusive.
- Migrate `hostname()` as a toy example, to show the idea.

Problem:  Resuming terminal process doesn't work with command in fish.
Solution: Send SIGCONT to the entire process group.

Use killpg() like what bash and zsh do on `fg`:
https://cgit.git.savannah.gnu.org/cgit/bash.git/tree/jobs.c?id=637f5c8696a6adc9b4519f1cd74aa78492266b7f#n3928
https://sourceforge.net/p/zsh/code/ci/77045ef899e53b9598bebc5a41db93a548a40ca6/tree/Src/jobs.c#l2674
https://sourceforge.net/p/zsh/code/ci/77045ef899e53b9598bebc5a41db93a548a40ca6/tree/Src/signals.c#l538

Install fish on CI to test this.

"Error" in error messages is redundant. Just provide the context, don't
say "Error ...".

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.