Integrate the JRuby extension

[knu]

This work is based on #21. Ruby engine specific parts now reside in their respective ext/* directories, while keeping the common parts in lib/.

[eregon]

👍 looks good to me.

[ahorek]

👍 LGTM

I don't think that the final java gemfile should be shipped with C sources - ext/digest/** and vice versa. But it's a detail.

[knu]

@ahorek You have a point. I added a commit to address that.

[knu]

I've squashed some commits. I think I can merge this later today. Thank you all for taking a look!

[nobu]

Some commits, CI changes and dropping ext/openssl, seem better to be separated from this issue.

[nobu]

I think that the @eregon's idea, mapping .so to .jar in JRuby, is better for other libraries too.

[eregon]

I think that the @eregon's idea, mapping .so to .jar in JRuby, is better for other libraries too.

It would be good to get JRuby's feedback on that, I opened jruby/jruby#6865