Split backend/common in backend/ and common/

This directory used to contain both the state tracking code for the
backends, and the common utilities that could be used both by the
backends and the rest of the code. Things are now:

 - src/common is utility code for the whole repo
 - src/backend contains libNXT's code
 - src/utils is utility code that we don't want in libNXT

This commit also changes all includes to use global paths from src/
bacause it had to touch a bunch of #include statements anyway.

src/common/Math.h

@@ -0,0 +1,43 @@
+// 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 COMMON_MATH_H_
+#define COMMON_MATH_H_
+
+#include <cstddef>
+#include <cstdint>
+
+namespace backend {
+
+    // The following are not valid for 0
+    uint32_t ScanForward(uint32_t bits);
+    uint32_t Log2(uint32_t value);
+    bool IsPowerOfTwo(size_t n);
+
+    bool IsAligned(const void* ptr, size_t alignment);
+    void* AlignVoidPtr(void* ptr, size_t alignment);
+
+    template<typename T>
+    T* Align(T* ptr, size_t alignment) {
+        return reinterpret_cast<T*>(AlignVoidPtr(ptr, alignment));
+    }
+
+    template<typename T>
+    const T* Align(const T* ptr, size_t alignment) {
+        return reinterpret_cast<const T*>(AlignVoidPtr(const_cast<T*>(ptr), alignment));
+    }
+
+}
+
+#endif // COMMON_MATH_H_