[jvm] global clean

[tisonkun]

I see the JVM backend is a bit messy and willing to do a global clean, make this pull request to listen if this is supported and run travis tests

[tisonkun]

travis fails on a missing return, would send a fix with next commit.

[jnthn]

Does this achieve anything besides making git blame less useful and cluttering final all over the place on stuff that's effectively final anyway? :/

[tisonkun]

For the former, yes, I mix up layout adjustment and code refactor. They should have been two topic. Sorry.
For the latter, it is a code style thing. Whether place final to guard variables are intended to be final, or, use effectively final semantic, save the final word and confirm a variable is final by analyze.

[AlexDaniel]

@tisonkun your commits were reverted here: rakudo/rakudo#2406 (comment)

Please take a look and see if they can be reintroduced again.