[jrubyc] respect Ruby visibility rules (do not generate private methods)

[kares]

jrubyc --java generation updated to be a little smarter - trying to respect Ruby's visibility rules.

... private, protected, public as well as their private :method, ... counterparts now seamlessly 'work'

private Ruby methods (without an explicit java_signature '...' which is allowed to force generation) are no longer generated. also protected methods map to Java protected visibility by default.

(kind of) resolves #4469 (still undecided about what to do with with xxx? and yyy! methods)

[kares]

@headius maybe this is good to go for 9.1.9 - to move some of the jrubyc --java users forward (#4469) ?

[headius]

The question is whether the old behavior could be considered a feature by some, a feature that we are taking away. I don't feel that way though, so I am on board with including it.

We may want to consider moving jrubyc to a preinstalled gem for 9.2, so it can be updated independently.

[kares]

OK ... I'm fine with leaving it for 9.2 as well - let's have @enebo to 🎱 for us