expr: New module for Boolean expressions on fields, for use in OVN.

expr: New module for Boolean expressions on fields, for use in OVN.

Known weaknesses:

    * String fields can't be converted to flows yet.  A subsequent commit
      will fix this.

    * Flows aren't optimal in some situations likely to be common.
      I don't think either of those is a reason not to commit this;
      this is a solid base to build on.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>