IpPrefix improvements.

1. Allow IpPrefixes to be created from strings. In order to do
   this, factor out the code from LinkAddress which already does
   this to a small utility class in NetworkUtils.
2. Truncate prefixes on creation, fixing a TODO.
3. Add a toString method.
4. Write a unit test.

While I'm at it, make RouteInfoTest pass again, and convert it
to use IpPrefix instead of LinkAddress.

Change-Id: I5f68f8af8f4aedb25afaee00e05369f01e82a70b