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/P4Lang/Program.swift

@@ -28,7 +28,7 @@ public struct ExpressionStatement {
 public struct Program {
   public var types: [P4Type] = Array()
   public var externs: [P4Type] = Array()
-  public var instances: [P4QualifiedType] = Array()
+  public var instances: [P4Value] = Array()
 
   /// Type of closure for filtering results from ``Program/InstancesWithTypes(_:)``
   public typealias TypeFilter = (P4QualifiedType) -> Bool
@@ -36,7 +36,7 @@ public struct Program {
   public typealias DataTypeFilter = (P4Type) -> Bool
 
   /// Retrieve global instances in the compiled P4 program.
-  public func InstancesWithTypes() -> [P4QualifiedType] {
+  public func InstancesWithTypes() -> [P4Value] {
     return self.instances
   }
 
@@ -52,9 +52,9 @@ public struct Program {
   ///
   /// @Snippet(path: "use-program-instanceswithtypes", slice: "include")
   ///
-  public func InstancesWithTypes(_ filter: TypeFilter) -> [P4QualifiedType] {
+  public func InstancesWithTypes(_ filter: TypeFilter) -> [P4Value] {
     return self.instances.filter { instance in
-      filter(instance)
+      filter(instance.type())
     }
   }
 
@@ -112,8 +112,8 @@ public struct Program {
   }
 
   public func find_parser(withName name: Identifier) -> Result<Parser> {
-    for instance in self.instances {
-      guard let parser = instance.baseType() as? Parser else {
+    for instance in self.types {
+      guard let parser = instance as? Parser else {
         continue
       }
       if parser.name == name {