feat: Make types accessible by name in `S`

[jceb]

Currently, S.env provides the environment with all defined types, however there's no way of directly accessing the types by name. Functions like S.is need a reference to a type to work properly so the proposal is to make types accessible by name from S, e.g. via S.types.TYPE or via refactoring S.env into a map so S.env.TYPE can be used.

[davidchambers]

Turning S.env into an object or a map is a great idea!