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/P4Compiler/Expression.swift

@@ -51,7 +51,8 @@ extension TypedIdentifier: CompilableExpression {
           sourceLocation: node.toSourceLocation(), withError: "Cannot find \(node.text!) in scope"))
     }
 
-    return .Ok(TypedIdentifier(name: node.text!, withType: type))
+    /// TODO: If there is a value here, then we can make this a compile-time constant!
+    return .Ok(TypedIdentifier(name: node.text!, withType: type.0))
   }
 }