Revert "Math: don't allow *::data() to be called on rvalues."

The pointer conversion can be done only explicitly, thus the users will
always know what they are doing. With that change, perfectly valid
things like this couldn't be done (the result of a + b is kept until the
semicolon):

    Vector3 a, b;
    void foo(float* data);
    foo((a + b).data());

Moreover this conversion wasn't even properly tested, leading to issues
as in mosra/corrade@781f5df38a7b1b366f3de477dd5fe641eca6ed20.

This reverts commit add989703e.

src/Math/RectangularMatrix.h

@@ -164,20 +164,8 @@ template<std::size_t cols, std::size_t rows, class T> class RectangularMatrix {
          *
          * @see operator[]
          */
-        T* data()
-        #ifndef CORRADE_GCC47_COMPATIBILITY
-        &
-        #endif
-        { return _data[0].data(); }
-
-        /** @overload */
-        constexpr const T* data()
-        #ifndef CORRADE_GCC47_COMPATIBILITY
-        const &
-        #else
-        const
-        #endif
-        { return _data[0].data(); }
+        T* data() { return _data[0].data(); }
+        constexpr const T* data() const { return _data[0].data(); } /**< @overload */
 
         /**
          * @brief %Matrix column

src/Math/Vector.h

@@ -184,20 +184,8 @@ template<std::size_t size, class T> class Vector {
          *
          * @see operator[]()
          */
-        T* data()
-        #ifndef CORRADE_GCC47_COMPATIBILITY
-        &
-        #endif
-        { return _data; }
-
-        /** @overload */
-        constexpr const T* data()
-        #ifndef CORRADE_GCC47_COMPATIBILITY
-        const &
-        #else
-        const
-        #endif
-        { return _data; }
+        T* data() { return _data; }
+        constexpr const T* data() const { return _data; } /**< @overload */
 
         /**
          * @brief Value at given position