export & document the safe-castable numpy sctype aliases

[jorenham]

They're currently called e.g. integer_co and f64_co and live in numpy/_to.py. I'm not very fond of the *_co suffix, as it's easy to confuse with the *_co suffix that's conventionally used by covariant TypeVars. Something like a Co*, As*, or To* prefix would have my preference, as well as camel-cased names.