Add clamp function

Request in issue 10039

http://d.puremagic.com/issues/show_bug.cgi?id=10039
Adjust clamp signature and tests

signed is now deduced over unsigned, added small documented unittest
instead of full block
Add in contract for clamp()

lower bound should not be larger than higher one
Mention 'clamp' in the cheatsheet
Merge pull request #2327 from Dicebot/remake-1360

PR 1360 revived: Add clamp function