Restructuring the compiler with intermediate data structures

[vvp]

Current iteration of js2amb works as a proof-of-concept, but the way it's designed will make the path forward hard if not impossible. This PR restructures the whole compiler, and introduces intermediate data structures for different compilation steps when parsing JS to Ambients.

Before this, parsing JS with Esprima and turning that directly to algebra-strings quickly ends up in if/else-heavy code, that doesn't cover all the possible combinations of JS expressions to corresponding Ambients primitives (basically, it's not compositional). This PR changes this by introducing few more AST-types to orchestrate the compilation steps better and enable compiler to do type checking and optimizations without hardcoding.

This is still work-in-progress, but wanted to let you know what does it look like and where is this going 🙂