Avoid dead code warnings in generated code.

If we blindly splicing `{..$stats, ..$generatedCode}`, and the last
expression in `$stats` is of type `Nothing`, we'll incur a dead
code warning when typechecking the block.

This commit:
  - introduces a helper method to augment user-written stats with
    synthetic code
  - Emit a try/finally in that code (so we advance the state, even if we
    are about to exit the state machine in the async-block global
    exception handler
  - Hide `Nothing` typed expressions from the dead code analysis
    by wrapping them in an `expr: Any`

Fixes #150 (the part reported in the comments, not the original ticket.)

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