Constraints for simple tasks

[afs]

Having specific forms for simple cases:

• (users) it makes it clearer to see what a constraint does; less error-prone.
• (implementers) helps implementation/optimization, avoiding analysis regexes; for light-weight implementations.

e.g. for strings:

• sh:strContains
• sh:strStarts, sh:strEnds

This can already be performed by sh:pattern but being able to say them without regexs is easier.

eg. for URIs:

• some way to say "in namespace" (effectively, URI prefix)