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/Statement.swift

@@ -255,7 +255,7 @@ extension VariableDeclarationStatement: CompilableStatement {
         // Context with updated names to include the newly declared name.
         context.update(
           newInstances: context.instances.declare(
-            identifier: parsed_variablename, withValue: declaration_p4_type))
+            identifier: parsed_variablename, withValue: (declaration_p4_type, .none)))
       )
     )
   }