* Prefer `reserve` rather than `fill constructor` in AS::map()
`fill constructor` constructs the vector with n elements and initializes each element with its default constructor. Instead, `reserve` just allocates memory, no element is initialized and put into the vector, so it's more effeicent. For example, after running the `result_type res(iterable.size());` statement, the `res` vector's size is equal to `iterable.size()`, but after running the `res.reserve(iterable.size());` statement, the `res` vector's size is 0.
* Add tests for ASLayoutSpecUtilities
