vim-patch:7.4.235

> Problem:    It is not easy to get the full path of a command.
> Solution:   Add the exepath() function.

> https://code.google.com/p/vim/source/detail?r=5ab2946f7ce560985830fbc3c453bb0f7a01f385

I think this should remain a WIP until #938 gets merged, but I thought it would be better to put this up rather than just sit on it. A few things might change before [RDY] anyway.
That's it! Thanks.

I dont think that adding `NULL` to `garray_spec.moon` would fit this PR, but that seems to be the trend.

```bash
$ cd test/unit
$ ag 'NULL =' 
garray_spec.moon
6:NULL = ffi.cast 'void*', 0

os/users_spec.moon
5:NULL = ffi.cast 'void*', 0

os/env_spec.moon
6:NULL = ffi.cast 'void*', 0

path_spec.moon
8:NULL = ffi.cast 'void*', 0
```