logical-fields: New header for logical field assignments.

logical-fields: New header for logical field assignments.

The original concept for "expr" and "actions" was that they should not need
to know anything about the mapping between physical and logical fields,
that instead everything should be provided via the symbol table.  In
practice this has proven difficult because a couple of actions need to know
about logical fields.  For now, it seems reasonable to put the logical
field mapping into a header of its own.  Later, maybe we'll figure out
whether there's value in a less leaky abstraction.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>