Problem:
- Not obvious which _meta/ are generated and which should be edited
  manually.
- The require guard (`error('Cannot require a meta file')`) is not
  consistently present in all meta files.

Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.

Problem:
The "gitsigns" plugin runs `vim.diff` in a thread (`uv.new_work`), but
`vim.diff` is nil in that context:

    Lua callback:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: bad argument #1 to 'decode' (string expected, got nil)
    stack traceback:
      [C]: in function 'decode'
      …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: in function <…/gitsigns.nvim/lua/gitsigns/diff_int.lua:29>
    Luv thread:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:63: attempt to call field 'diff' (a nil value)

Solution:
Revert the `stdlib.c` change (set `vim.diff` instead of `vim._diff`).