Upgrade ASM to v4, and use a CFG to correctly generate bytecode for try-catch blocks.

ASM4 is more strict about having all the try-catch blocks defined before their labels are visited; this means that the try-catch blocks should be generated (typically) as the first statement(s) in a (generated) method.

I do this work in 2 passes: 
- IRBuilder: builds an IR and adds edges between nodes of a try-catch block.
- StampTryCatch: Looks at the edges inserted in the previous pass and generates TryCatchBlock nodes and inserts them at the start of each new scope.