Mask values in a match according to its masks

A step towards match canonicalization.  When generating a match
structure, clear the bits outside of those indicated by the
masks.  Changes to test code to make consistent.

For 1.0, need only worry about IPv4 addrs.  Otherwise, use a simple
loop to iterative over bytes in the match.
Merge into master from pull request #44:
Mask values in a match according to its masks (https://github.com/floodlight/loxigen/pull/44)