Set errno in alloc functions and add further tests

Make malloc, calloc, realloc, aligned_alloc and memalign set errno upon failure (as specified by POSIX/glibc docs). Also add further tests to exercise all branches of these functions.

Note that this MR makes memalign stricter in behavior, since it will now fail upon non-power-of-two alignment inputs (as described in glibc documentation) rather than just wrapping the underlying allocator function.