Disentangle ujson from _lib/src/datetime

[jbrockmendel]

There are some non-trivial simplifications (and perf improvements) in tslibs.np_datetime available if ujson can be made to not call pandas_datetimestruct methods directly. Is there a viable way to make the ujson code go through the tslibs cython instead of through src/datetime?

[jbrockmendel]

@chris-b1 any idea how to do a c-equivalent of:

with tm.set_timezone(“UTC”):

? If that can be done then I think I have a fix for this issue.

[WillAyd]

@jbrockmendel what's the upside of doing this? I think potentially possible if we use the public keyword:

http://docs.cython.org/en/latest/src/userguide/external_C_code.html#public-declarations

But not sure on what it buys us as we'd just be adding an extra layer in between JSON and the date time conversion routines

[jbrockmendel]

IIRC the motivations were

• if np_datetime only needed to handle use cases getting called from within tslibs, then most of the non-nano code could be ripped out. I think there were some other simplifcations i had in mind, but dont remember them ATM.
• some hope that by eliminating the dependencies in setup.py we might speed up compilation and/or reduce the compiled size