Refactor Parser States

Refactor the class hierarchy for parser states so that
there is a parser state that acts more like a type and
one that acts more like a value (the latter, then, serves
as the base class for the other instantiated parser states).

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Tests/p4rseTests/ParserCompilerTests.swift

@@ -56,7 +56,7 @@ import P4Lang
 
   #expect(parser.states.count() == 1)
 
-  let state = try! #require(parser.states.find(withIdentifier: Identifier(name: "start")))
+  let state = AsInstantiatedParserState((try! #require(parser.states.find(withIdentifier: Identifier(name: "start")))))
   #expect(state.state == Identifier(name: "start"))
   #expect(state.statements.count == 1)
 }