vint: export string_of_seq
vint: replace FieldMap with pattern record
vint: consistently use portId in SDN_Types
vint: remove Int32m variant
vint: add VInt.get_int12

Used for casting vlan ids.
vint: replace field/fieldVal with explicit actions

Rather than having a fieldVal type that must accommodate all the
different bit widths of fields, instead create an action for each field,
which explicitly determines the bit width of its associated value.

After this commit, it's no longer necessary to expose VInt in a public
API. It will remain in the code as a means to assist in casting between
various bit widths.