Allow only part of a module to be imported

[dorchard]

We should extend imports to only allow certain definitions to be included, e.g.,

    import Prelude(if0)

This would be a relatively simple change, adding this syntax to parser, which would then get serialised into the AST and used to control what is included (but note this would also require chasing dependencies of if0).

[buggymcbugfix]

If the main itch this scratches for now is the speed of type checking, then perhaps caching the checker state per module would do that?