Desugar the head stmts all the way out of STMT_for, rather than stashing them in the node. That's only necessary for STMT_while.

Revert previous wrong-change on issue #106; was papering over wrongness in the runtime. Sad times. Closes #106 more correctly (and adds comment to explain).

Add fmt module, move out some common format helpers, add instruction-selection tracing and make selection use queues rather than list refs.