[Distributed] Drop class requirement from DistributedTargetInvocationDecoder

This allows us to avoid heap allocation and reference counting
when working with decoders. Also both (public and internal)
distributed target execution methods has been made generic over
a decoder type.

Author: xedin

stdlib/public/Distributed/DistributedActorSystem.swift

@@ -154,7 +154,7 @@ extension DistributedActorSystem {
   public func executeDistributedTarget<Act, ResultHandler>(
     on actor: Act,
     mangledTargetName: String,
-    invocationDecoder: InvocationDecoder,
+    invocationDecoder: inout InvocationDecoder,
     handler: ResultHandler
   ) async throws where Act: DistributedActor,
                        // Act.ID == ActorID, // FIXME(distributed): can we bring this back?
@@ -289,7 +289,7 @@ extension DistributedActorSystem {
       try await _executeDistributedTarget(
         on: actor,
         mangledTargetName, UInt(mangledTargetName.count),
-        argumentDecoder: invocationDecoder,
+        argumentDecoder: &invocationDecoder,
         argumentTypes: argumentTypesBuffer.baseAddress!._rawValue,
         resultBuffer: resultBuffer._rawValue,
         substitutions: UnsafeRawPointer(substitutionsBuffer),
@@ -309,10 +309,10 @@ extension DistributedActorSystem {
 
 @available(SwiftStdlib 5.7, *)
 @_silgen_name("swift_distributed_execute_target")
-func _executeDistributedTarget(
+func _executeDistributedTarget<D: DistributedTargetInvocationDecoder>(
   on actor: AnyObject, // DistributedActor
   _ targetName: UnsafePointer<UInt8>, _ targetNameLength: UInt,
-  argumentDecoder: AnyObject, // concrete type for `InvocationDecoder`
+  argumentDecoder: inout D,
   argumentTypes: Builtin.RawPointer,
   resultBuffer: Builtin.RawPointer,
   substitutions: UnsafeRawPointer?,
@@ -400,10 +400,10 @@ public protocol DistributedTargetInvocationEncoder {
 
 /// Decoder that must be provided to `executeDistributedTarget` and is used
 /// by the Swift runtime to decode arguments of the invocation.
-public protocol DistributedTargetInvocationDecoder : AnyObject {
+public protocol DistributedTargetInvocationDecoder {
   associatedtype SerializationRequirement
 
-  func decodeGenericSubstitutions() throws -> [Any.Type]
+  mutating func decodeGenericSubstitutions() throws -> [Any.Type]
 
 //  /// Ad-hoc protocol requirement
 //  ///
@@ -420,9 +420,9 @@ public protocol DistributedTargetInvocationDecoder : AnyObject {
 //  /// performs the actual distributed (local) instance method invocation.
 //  mutating func decodeNextArgument<Argument: SerializationRequirement>() throws -> Argument
 
-  func decodeErrorType() throws -> Any.Type?
+  mutating func decodeErrorType() throws -> Any.Type?
 
-  func decodeReturnType() throws -> Any.Type?
+  mutating func decodeReturnType() throws -> Any.Type?
 }
 
 @available(SwiftStdlib 5.7, *)