Allow disabling hashable dependency

[414owen]

This allows people to disable the Hashable instance, and dependency.
To keep compatibility, it's implemented as a flag that defaults to true.
I think that if async were to release a new major version, it would be good to implement this as a sublibrary instead.

[simonmar]

What's the rationale for this?

[414owen]

I have a project that I'm keeping quite lean, and at the moment async is one of the two dependencies that depends on hashable.

In general, minimizing dependency footprints gives us some small wins:

• Faster (first-time) compile times (eg in CI)
• Faster link times
• Smaller binary sizes
• Easier ghc upgrades (almost certainly not an issue with hashable)

Admittedly I might be scraping the bottom of the barrel trying to get rid of hashable... Feel free to close this.

[simonmar]

This seems to me a very narrow use case and not worth adding conditional compilation for. If there's a lot of demand then I'll reconsider. Closing for now.