meta-flow: New library for working with fields by id.

meta-flow: New library for working with fields by id.

OVS already has a fairly good set of functions for working with fields that
are known at compile time, but support for working with fields that are
known only at runtime is fairly limited (and fairly unneeded).  However,
with NXM identifiers becoming more and more widely used throughout Nicira
extensions, it's becoming corresponding more and more common to need to
refer to fields at runtime.  This new library represents a first attempt
at a systematic approach for doing so.