Remove hangul input support

Vim [documentation](http://vimdoc.sourceforge.net/htmldoc/hangulin.html), says
that hangul support is scheduled to be removed. I think it's safe to say we
don't want to support a feature even vim is considering removing.

Everything still compiles even after removing the header, so it's not being
used.

Before doing the initial import to neovim's repository, I had to tweak this
module to make it compile for terminal. It was a mistake that is now being
corrected.
Issue #311 - Clean up arabic.c, arabic.h, farsi.c, farsi.h. Tweaks to uncrustify.cfg.
Add another unit test to mch_can_exe.
Move mch_can_exe, executable_file to os/fs.c.

* Rename executable_file to is_executable.
Declare is_executable as static and remove its unit tests.

Testing the public interface mch_can_exe should suffice. Every former
test of is_executable has a counterpart in the tests of mch_can_exe.
Thus we can keep private things private.
Simplify mch_can_exe extracting is_executable_in_path.
Move definition of helper function below their usage.
Remove unused return values.
Add const specifiers, update comments, add assert.
Move os_unix.moon tests to os/fs.moon + clean tests.
Add uv_fs_req_cleanup call.
Add tests with absolute paths to mch_isdir.
Add to_cstr to test/unit/helpers.moon and fix respective unit tests.