Introduce a `ScopeAwareVisitor`

[ryangjchandler]

There are multiple scenarios where we want to traverse an AST and keep track of scope information, e.g. indexing, type-resolving & type-checking, and autocompletion.

Instead of duplicating a bunch of spaghetti code across various crates, we should instead introduce a ScopeAwareVisitor that handles the scope tracking for us with some default method implementations, then we can override the methods that we care about.

The information that we care about is:

• Current namespace
• Imported classes, functions, constants

Then some helper methods for getting scope info too:

• scope()
• scope_mut()
• current_namespace()
• qualify()

[ryangjchandler]

Closing in favour of #55.