Support length restrictions for operands

[Akron]

This was a request by Rainer:

It would be helpful to limit searches to spans of a certain length (e.g. sentences). To allow this for general spans, an operation like
toklen(<base/s=s>, 4, 12) (only sentences with a length between 4 or 12 tokens) may be useful.

As a result filter it may also be possible to support strlen(["der"][]{1,10}[word="Baum"], 16), that limits to all matches with exact 16 characters. (Though - this will be slow.)