The basic idea is that in the main mungedocs we run the entirefile and
create an annotated set of lines about that file. All mungers then act
on a struct mungeLines instead of on a bytes array. Making use of the
metadata where appropriete. Helper functions exist to make updating a
'macro block' extremely easy.
All munges now start with `<!-- BEGIN MUNGE:` and end with `<!-- END MUNGE:`.
This lets me (in a followup) filter them better to normalize contents during
verification of generated docs.
