Alias primitive return types in FFI to support more explicit documentation about those types

[cdata]

    Consider: FFI aliases for `NsString => char*`, `NsString|NsStringArray => char**` `NsBytes => [uint8_t]`. This could indicate not necessarily ownership, but the originator of the data (Rust or embedder) and the corresponding release function (e.g. `NsString` must call `ns_string_free()` rather than `free()`). Mostly applicable for return types, less so for method arguments (where we would probably want to keep the C primitive types)

Originally posted by @jsantell in #141 (comment)