With this change, it becomes possible to get the element type of named
slices, pointers, and channels.
This is a prerequisite to enable the common named struct types. There's
more to come.
This commit fixes the following issue:
https://github.com/tinygo-org/tinygo/issues/309
Also, it prepares for some other reflect-related changes that should
make it easier to add support for named types (etc.) in the future.
This commit makes sure all Go types can be encoded in the interface type
code, so that Type.Kind() always returns a proper type kind for any
non-nil interface.
