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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.
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