dillon1000/react

More precise handling of const/reassign in ssa form

Addressed a TODO from the previous PR. When we enter SSA form, when we rewrite 
variable reassignments we currently change the identifier but leave the kind of 
the lvalue alone; technically we should convert from Reassign to Const. After 
doing that, it's easier to correctly update when we leave SSA form, we can 
convert just a subset back into let/reassign (but leave most things alone as 
const).