Make co_reduce thread safe

[zbeekman]

Probably will get done with #172

The problem lies in global function pointers

[zbeekman]

@vehre I just wanted to confirm that I didn't miss anything and that your PR #332 did indeed resolve this issue

[vehre]

No, threadsafety is not addressed by

[vehre]

#332. Sorry, fingers to big for smartphone.

[zbeekman]

Ok my mistake, I thought I saw a new strict encapsulating the procedure pointer but I mostly just tested it for fixing #172 and did a quick review of sources. Thanks for fixing #172 though, that's huge! 💯