Using A-normal form for intermediate representation in JIT

Currently the JIT uses Java AST as intermediate representation, which is low-level (compared to ANF) and does not encapsulate a lot of higher-level program information. We should consider basing the IR on something like ANF (as suggested by Tesla) so that 1) it is more backend-agnostic so we can revive the Java source codegen 2) better JIT support for `let` can be implemented.