Rename and refactor mch_FullName.

* Rename mch_FullName to mch_full_name to match the style guide.
* Add mch_full_dir_name, which saves the absolute path of a given
directory relative to cwd into a given buffer.
* Add function append_path, which glues together two given paths with a
slash.
* Adapt moonscript coding style to the tests.
Correct a few parentheses.
Apply small refactor to unit tests

Redefine macro constants as enums in the ffi and import those in their
respective test modules.
Assert that vim_strsize and vim_strnsize gets a non-null parameter.