dillon1000/react

TryStatement: maybe-throw terminal for per-instruction throw points

Adds a "maybe-throw" terminal which represents the possibility that the block 
may or may not throw, and can either continue forward or exit to an exception 
handler (`catch`). Also updates HIRBuilder to track the current mode, and when 
inside a try block to wrap every instruction inside a basic block that ends in a 
maybe-throw. 

So far this code isn't used yet, so doesn't affect output.