dillon1000/react

[rust] New data model for HIR

This change is motivated by starting to explore porting EnterSSA to Rust. It's a 
good medium complexity pass and quickly demonstrates why a direct port of our 
existing data model and algorithms won't work so well. For examples just these 
first lines at the top of the transform create multiple references to the 
function body/blocks: 


https://github.com/facebook/react-forget/blob/58da89888eabde17ede649d348b307ee01fc15e1/forget/packages/babel-plugin-react-forget/src/SSA/EnterSSA.ts#L230-L231 

But more generally it's always felt wrong that instructions have an LValue that 
isn't really used. So here i'm exploring making operands a newtype index into a 
single instructions array (shared for the entire function), and using different 
types for identifier references in SetLocal and LoadLocal. Incidentally this 
also declutters the printed HIR quite a bit. 

I'm not going to land this until i actually finish enter_ssa() and some other 
passes. We definitely need to balance fidelity to the existing code (to 
facilitate porting) with using idiomatic Rust (to facilitate porting in the 
sense of not fighting the borrow checker).