dillon1000/react

[rust] Data structures for semantic analysis

This is two things: 

* A toy semantic analysis that handles a tiny subset of JS, including labeled 
statements, labeled break/continue, and variable 
declaration/reference/reassignment. This only exists as a way to prove out the 
API for the more important bit: 

* More importantly, this defines a data model for the semantic analysis results 
and an API for building up the semantic analysis. 

Subsequent diffs will replace the first bit (toy analysis impl), while keeping 
the second part.