In P4, parsers are considered types. Those parsers are instantiated.
The instantiated parsers are values. Previously, gp4 treated a parser
type and a parser value as identical. This PR makes that difference
clear _and_ sets the stage for the future.
TODO: Make the same distinction between control and action types and
values.
Signed-off-by: Will Hawkins <hawkinsw@obs.cr>
Parameterize Runtime (nee ParserRuntime) so that it can run anything
that is LibraryCallable. TODO: Refactor its API -- left mostly as
it was because changing unit tests will be laborious.
Will Hawkins