WIP: Date/DateTime support through the Time module

As discussed in #3524 and @karbarcca/Datetime#12.

This is most of the Datetime package reworked to simplify, speed up, and polish some functionality (e.g. date parsing). Notably missing is full Timezone support, which I've also been reworking. I figured I'd submit this PR and things can get ironed out a little more before all the Timezone code is finalized. I also have the Datetime manual and function reference which can be folded in fairly easily once things settle down.

I've included a few RFC, TODOs, and some comments on certain pieces that I'd appreciate feedback on. Also any feedback on how to fold this into Base would be appreciated as I've never pushed anything like this before.

Cheers!