CLN: use float64_t consistently instead of double, double_t
remove some commented-out or otherwise unused code

disable boundscheck/wraparound in a couple places in tslib where it is safe

add `const` modifiers to some of the memoryview functions so they don't raise if we ever pass read-only arrays to them

standardized NPY_NAT as always being the `cdef int64_t` version and `iNaT` as being the python-namespace version

remove duplicated NaN/nan constants

remove non-standard imports of np.nan in some test files