compiler, language, runtime: Separate Parser Type From Instances

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>

Tests/p4rseTests/ParserCompilerTests.swift

@@ -46,7 +46,7 @@ import P4Lang
     parser main_parser() {
        state start {
            true;
-           transition start;
+           transition accept;
        }
     };
     """
@@ -56,8 +56,8 @@ import P4Lang
 
   #expect(parser.states.count() == 1)
 
-  let state = AsInstantiatedParserState((try! #require(parser.states.find(withIdentifier: Identifier(name: "start")))))
-  #expect(state.state == Identifier(name: "start"))
+  let state = (try! #require(parser.states.find(withIdentifier: Identifier(name: "start"))))
+  #expect(state.getName() == Identifier(name: "start"))
   #expect(state.statements.count == 1)
 }