Release 1.7.2 of bmv2

Bug fixes since 1.7.1:

• fix major bug in ternary table implementations (lookup results were wrong after deleting an entry because of invalid handle computation)
• fix typo in P4Objects code which sets arith for union fields
• remove statements with side-effects from asserts, in case code is compiled with NDEBUG
• fix ByteContainer::to_hex to not use operator [] to get an "iterator" to the end of the vector; in debug mode, clang perform bound checking even for operator []