Add TypeSpecifier
Add tests for TypeSpecifier backend conversion
Rename Alias, TypeAlias, FunctionValueAlias

- Alias → ImportAlias
- TypeAlias → ImportTypeAlias
- FunctionValueAlias → ImportFunctionValueAlias

in order to avoid
- confusion
- a name conflict with alias declarations

See also #46.
Visitor: Sort visit() methods

I’m not sure how long they’ve been out of order… but as soon as
something is wrong, source-gen amplifies it with every added type :-/