PERF: cython optimizations
cython was emitting some warnings about untyped array lookups in `tslibs.conversion`; added types to those

`arr.fill(val)` is a python call, while `arr[:]` is a C call, so replaced all of those

modernized an old non-python for-loop, some other small cleanups along the way.

We're running out of available optimizations/cleanups in these files.  One thing I'd be interested in changing (but I'm not confident enough to do) is removing uses of `double` and `double_t`, which I _think_ are equivalent to `float64_t`.  But I'm not sure if that is architecture or platform dependent or something.