Custom Parser and Printer

[webmiche]

It would be very cool to have easy support for defining custom parser and printer. In particular, some of the tree-based code becomes very ugly, there defining custom syntax like the following would be very helpful:

op_with_regions()
with first_region = {
  *region content*
}
with second_region = {
  *region content*
}

I might just be out of the loop on these things, is there some support for this already, @math-fehr ?

[math-fehr]

I'll make a quick prototype! AFAIK, Martin had a version as well in the Linalg PR.