Failing async-io specs

[eregon]

From #1646
cc @ioquatix

First issue is rb_enc_sprintf not yet implemented, I have a trivial fix for that.
With that fixed, specs no longer hang and I get

111 examples, 20 failures

Some of these issues were already mentioned by @aardvark179 in #1646 (comment)

[ioquatix]

Nice, I look forward to trying this out.

[eregon]

rb_enc_sprintf merged in a986766.

[eregon]

I removed extra public methods on IO in 1a6aa8d.
Still needs some Socket fixes to handle exception: false, going through CI.

[eregon]

And Socket APIs now support exception: false in fa9863d.
That's all that's required to pass all async-io's specs when running on my machine.
When 20.0.0-beta2 is released, we can remove TruffleRuby from allowed failures in TravisCI.
@ioquatix Thanks for the help getting them passing.

[ioquatix]

Awesome!

[ioquatix]

When can you release 20.0.0-beta2?

[eregon]

@ioquatix Not sure yet, I'll ping you here when it's released.