mbe: Refactor transcription

Introduce `MacroTcbCtx` that holds everything relevant to transcription.
This allows for the following changes:

* Split `transcribe_sequence` and `transcribe_metavar` out of the
  heavily nested `transcribe`
* Split `metavar_expr_concat` out of `transcribe_metavar_expr`

This is a nonfunctional change.

Author: tgross35