Move the Codecs to NIOCore (#1927)

Motivation:

Our basic Channel Handlers don't need to be in NIO anymore: they aren't
realistically tied to that code. So we can move them to NIOCore.

Modifications:

- Move Codecs to NIOCore
- Move SingleStepByteToMessageDecoder to NIOCore.

Result:

Even more stuff in NIOCore

Author: Lukasa

Sources/NIO/Codec.swift renamed to Sources/NIOCore/Codec.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the SwiftNIO open source project
 //
-// Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
+// Copyright (c) 2017-2021 Apple Inc. and the SwiftNIO project authors
 // Licensed under Apache License v2.0
 //
 // See LICENSE.txt for license information
@@ -14,7 +14,8 @@
 
 //
 import XCTest
-@testable import NIO
+import NIO
+@testable import NIOCore
 
 public final class NIOSingleStepByteToMessageDecoderTest: XCTestCase {
     private final class ByteToInt32Decoder: NIOSingleStepByteToMessageDecoder {

Sources/NIO/SingleStepByteToMessageDecoder.swift renamed to Sources/NIOCore/SingleStepByteToMessageDecoder.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the SwiftNIO open source project
 //
-// Copyright (c) 2017-2019 Apple Inc. and the SwiftNIO project authors
+// Copyright (c) 2017-2021 Apple Inc. and the SwiftNIO project authors
 // Licensed under Apache License v2.0
 //
 // See LICENSE.txt for license information
@@ -13,7 +13,8 @@
 //===----------------------------------------------------------------------===//
 
 import XCTest
-@testable import NIO
+@testable import NIOCore
+import NIO
 import NIOHTTP1
 @testable import NIOWebSocket