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/ExternDeclarations.swift

@@ -89,10 +89,10 @@ public struct Return6: P4FFI {
       simple_parser_declaration, withGlobalInstances: .none, withGlobalTypes: .none,
       withFFIs: [externally]))
 
-  let runtime = try #UseOkResult(P4Runtime.Runtime<InstantiatedParserState, P4Lang.Parser>.create(program: program))
+  let runtime = try #UseOkResult(P4Runtime.Runtime<InstantiatedParserState, P4Lang.ParserValue>.create(program: program))
   let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(AsInstantiatedParserState(state_result) == P4Lang.accept)
+  #expect(state_result == P4Lang.accept)
 
 }
 
@@ -117,8 +117,8 @@ public struct Return6: P4FFI {
       simple_parser_declaration, withGlobalInstances: .none, withGlobalTypes: .none,
       withFFIs: [externally]))
 
-  let runtime = try #UseOkResult(P4Runtime.Runtime<InstantiatedParserState, P4Lang.Parser>.create(program: program))
+  let runtime = try #UseOkResult(P4Runtime.Runtime<InstantiatedParserState, P4Lang.ParserValue>.create(program: program))
   let (state_result, _) = try! #UseOkResult(runtime.run())
 
-  #expect(AsInstantiatedParserState(state_result) == P4Lang.reject)
+  #expect(state_result == P4Lang.reject)
 }