Importer/GlobalEnv/ResEnv rectification campaign

[favonia]

• [ ] cache the locations of redlib files.
• [x] unify the API for data declarations and normal declarations.
• [x] unify the tags in GlobalEnv and ResEnv.
• [ ] detect incorrect usage of redlib files.
• [ ] make Elaborator use Importer, not the other way around.
• [ ] mark the end of a module, and clean up of private bindings then