Functions will be invoked with the matched text, and their result will be
coerced to a string and used as the new replacement text.
This also allows passing non-function, non-byteviewable values, which will be
converted into strings during replacement (only once, and only if at least
one match is found).
Comparison between different bracket and normal tuples
will now take into account the delimiter type. This solves strange
non-locality issues in the compiler due to this false equality, and is
more consistent with Janet's otherwise strong equality philosophy.
