More memory efficient `import` statements

[FilipToth]

Background and Motivation

Currently, we mix all syntax trees into a collection and when an import statement referencing the syntax tree we insert the tree into another collection. That's not very efficient.

Proposed Solution

I propose we search all global statements for imports in the BindGlobalScope method and then see if the file name referenced in the import corresponds to a syntax tree. If it does, we'll add it to the _importedTrees collection and allow it's child methods to be referenced. This will also illegalize import statements outside the global scope.

[FilipToth]

This has now become a duplicate of #69.