[IDEA] Migrate to Bordeaux-Threads APIv2

[adlai]

Is your idea related to a problem?

The old API is less likely to receive bugfixes in the future.

Describe the solution you'd like

Originally, ChanL was supposed to have its own optimised code for each implementation; this was abandoned after some sophomoric mistake with CCL. Using BT2 is a reasonable compromise, because most any library doing anything portably parallel already depends on BT directly or otherwise.

Describe alternatives you've considered

• depression tells me not to do anything
• anxiety tells me inaction is worse than planning

Additional context

Official blog post