Resolve platform dependent cstr point types for menu API

[tehsmeely]

Resolves the types being used to represent cstring pointers in the menu API
being platform dependent.

Speficially I originally wrote it using i8 but it turns out other platforms use u8.
See the docs for more

The type of the returned pointer is *const c_char, and whether it’s an alias for *const i8 or *const u8 is platform-specific.

So we never force this into one of the two, and just use the ffi::c_char type as returned by CString::as_ptr

This refers to
#79
and
#82