document from_native and to_native
core: fix typos in rstring.rb
templates: properly handle methods whose type is converted to a non-FFI type
templates: give access to the *Struct type from the user-visible type

This is required for some of the generic to_native/from_native code
as the *Struct types are the ones that FFI really accesses.
add a generic typelib-to-rbind bridge