Metal: split non-trivial objects in their own file.

No functional changes intended, but there are a couple additional
cleanups:
 - Use anonymous namespaces instead of static functions
 - Don't store an extra Device pointer in objects
diff --git a/src/backend/metal/ShaderModuleMTL.h b/src/backend/metal/ShaderModuleMTL.h
new file mode 100644
index 0000000..6b7c3f7
--- /dev/null
+++ b/src/backend/metal/ShaderModuleMTL.h
@@ -0,0 +1,45 @@
+// Copyright 2017 The NXT Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef BACKEND_METAL_SHADERMODULEMTL_H_
+#define BACKEND_METAL_SHADERMODULEMTL_H_
+
+#include "common/ShaderModule.h"
+
+#import <Metal/Metal.h>
+
+namespace spirv_cross {
+    class CompilerMSL;
+}
+
+namespace backend {
+namespace metal {
+
+    class ShaderModule : public ShaderModuleBase {
+        public:
+            ShaderModule(ShaderModuleBuilder* builder);
+            ~ShaderModule();
+
+            id<MTLFunction> GetFunction(const char* functionName) const;
+            MTLSize GetLocalWorkGroupSize(const std::string& entryPoint) const;
+
+        private:
+            id<MTLLibrary> mtlLibrary = nil;
+            spirv_cross::CompilerMSL* compiler = nullptr;
+    };
+
+}
+}
+
+#endif // BACKEND_METAL_SHADERMODULEMTL_H_