Fancy coroutines

[Ericson2314]

First commits make internal changes. Final adds new stuff. Everything works but needs to be cleaned and documented---opening this now for feedback during that cleaning and documenting. LMK if leaving out the last commit makes more sense as first step.

CC @whitequark

[whitequark]

@nathan7 So, I've looked at this PR again.

• It adds 1300 lines of extremely convoluted code whose only significant purpose is to avoid a memcpy.
• We don't export the unsafe interface at all, anymore.

[Ericson2314]

@whitequark the first part is that's just not true :(. Not sure what you mean by the second. I'll rebase on top of the latest changes.

[edef1c]

(closing this mainly for inactivity)