Certain data types are forbidden in casts
Rewrite unnecessary casts
Tests for cast on LHS and attr on RHS
Remember which tests failed

so we can have "fail 1" and "fail 4", instead of "fail" and "fail"
Remove unnecessary casts in switch / case evaluation
better debug messages for casts
When evaluating a cast, avoid vp_aprint() if possible

This means doing more work on the raw data, but that will avoid
a lot of unnecessary string print and parsing
Remove leading "0x" when expanding octet types

Which is a simple patch now that the evaluation of casts avoids
printing && parsing of attributes
Check for unknown attr on the RHS
Checks for casts to differing sized attributes
Allow integer64 to be converted to ethernet
add const to cast