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>

Sources/P4Runtime/Runtime.swift

@@ -37,18 +37,19 @@ public struct Runtime<U, T: LibraryCallable<U>>: CustomStringConvertible {
   /// Create a parser runtime from a P4 program
   public static func create(
     program: P4Lang.Program
-  ) -> Result<Runtime<InstantiatedParserState, Parser>> {
+  ) -> Result<Runtime<InstantiatedParserState, ParserValue>> {
     return Runtime.create(program: program, withGlobalValues: .none)
   }
 
   public static func create(
     program: P4Lang.Program, withGlobalValues initial: VarValueScopes?
-  ) -> Result<Runtime<InstantiatedParserState, Parser>> {
+  ) -> Result<Runtime<InstantiatedParserState, ParserValue>> {
     return switch program.starting_parser() {
     case .Ok(let parser):
       .Ok(
-        P4Runtime.Runtime<InstantiatedParserState, Parser>(
-          callable: parser, withGlobalValues: initial))
+        P4Runtime.Runtime<InstantiatedParserState, ParserValue>(
+          callable: parser.instantiate(Identifier(name: "starting_parser")),
+          withGlobalValues: initial))
     case .Error(let error): .Error(error)
     }
   }