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/Common/Execution.swift

@@ -257,6 +257,12 @@ public typealias VarValueScope = Scope<P4Value>
 /// Scopes that resolves variable identifiers to their values.
 public typealias VarValueScopes = Scopes<P4Value>
 
+/// A scope that resolves variable identifiers to their values.
+public typealias StaticVarValueScope = Scope<(P4QualifiedType, P4Value?)>
+
+/// Scopes that resolves variable identifiers to their values.
+public typealias StaticVarValueScopes = Scopes<(P4QualifiedType, P4Value?)>
+
 /// Indicate the control flow result of a particular statement.
 public enum ControlFlow {
   case Next

Sources/Common/Protocols.swift

@@ -52,7 +52,7 @@ public protocol EvaluatableLValueExpression: EvaluatableExpression {
   func set(
     to: P4Value, inScopes scopes: VarValueScopes, duringExecution execution: ProgramExecution
   ) -> Result<(VarValueScopes, P4Value)>
-  func check(to: EvaluatableExpression, inScopes scopes: VarTypeScopes) -> Result<()>
+  func check(to: EvaluatableExpression, inScopes scopes: StaticVarValueScopes) -> Result<()>
 }
 
 public protocol ProgramExecutionEvaluator {