A couple micro optimizations

add faster npy_mul_with_overflow_intp and use it in PyArray_NewFromDescr and PyArray_OverflowMultiplyList
makes the former about two times faster according to perf.

improve _IsAligned, probably no measuable impact, but unifies it with raw_array_is_aligned.

improve test speed by making one faster and moving test_memmap_roundtrip to slow tests.