Refactored rewriting and passes

[Hugobros3]

• Introduces a Rewriter base class that subsumes over all instances of rewriting previously found in Thorin.
• Introduces a standard interface for rewriting nominal node (rewrite head then bodies in two steps) and make Continuations use it
• Refactored codegen_prepare as a rewriting pass
• Refactored PE to use the new rewriter for instantiating filters
• Refactored the mangler so it uses the new rewriter, and gave it some love
• Made Importer a rewriter subclass, and gave it some optimisation powers

Testing results

Artic tests pass, Rodent builds and runs, Stincilla tests build and run

[Hugobros3]

Update: a clean compile seem to have resolved stincilla build issues.