dillon1000/react

TryStatement: partial lowering

Implements the core lowering logic for try/catch: 

* Inside of the `try` block, we use the new HIRBuilder mode to wrap every 
instruction in a separate basic block with a maybe-throw terminal 

* We emit a 'try' terminal for the try/catch itself 

For basic examples this already works correctly. But we don't handle catch 
clause params yet.