Deep Extension Resolution

DEEP RESOLVE

Significantly improved LeafAST resolution
Syntax items now recursively rebuild themselves to allow deeply-nested #extend tags to properly resolve
Simplify LeafAST resolution steps, correct tail recursion
Adds appropriate test cases

This is an ugly but nonetheless functional update to the referenced template resolution; ASTs will now recursively examine ALL Syntax objects during loading and resolution to inline templates no matter how deeply buried.

I used tons of inouts on associated value Enums, don't @ me. This is not ideal in terms of code beauty but it does what it purports to.

Resolves #50

vapor / leaf-kit

Deep Extension Resolution #52

DEEP RESOLVE