Avoid blanket import from GHC.Conc

[edsko]

Corresponding ticket for async itself at simonmar/async#153.

[Bodigrim]

@amesgen if you get a moment to fix WASM CI job, it would be most appreciated. (I assume it's just a hash change, but cannot verify AFK)

[Bodigrim]

@edsko could you please rebase?

Any particular motivation for this change? Why GHC.Conc only?

[edsko]

@Bodigrim Have rebased. As for motivation: GHC.Conc exports some stuff that Control.Concurrent.STM also exports, which leads to name clashes when using a version of stm that doesn't straight up re-export stuff from GHC.Conc but does something slightly different (such as specifically https://github.com/edsko/stm-debug).

[Bodigrim]

Thanks!