Unparsed sections

[Howy Perrin]

For performance reasons and/or decomposition ergonomics, maybe it would be a good idea to support sections where the definition of what is allowed uses normal Rope types, but during parsing that section is only validated rather than actually decomposed.

I think this may make sense as there are often parts of an identifier where we care that it matches a particular format, but we don't need de-composed parts of it for later usage, and it would save the need to .write out a subsection to be able to use it as a String.