Allow type definitions to be exported.

[zaifir]

Currently, there is no way for a module to export type definitions for public use. (i.e. if a module M defines a type T, there is no way for programs that import M to make use of T; they must use * instead). This severely limits the usefulness of the (chicken type) system, since type checking cannot be used on types outside of the CHICKEN core.

I'd propose an export-like form, e.g. (export-type TYPE ...) which makes type definitions created with define-type available beyond the scope of the containing module.