feat(java): overhauled structs with native implementation, builders, ...

[bmaizels]

Overhauled structs with native implementation, builders, equals and hashCode and creation from JSII.

Fixes #525

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Pull request has been modified.

[spfink]

Other questions/thoughts:

Would be nice if there was imports and no fqcn
Can we get toBuilder() methods?

[RomainMuller]

@bmaizels I've suspended the auto-merging of this PR so you can go through @spfink's suggestions and decide what you do with them... For some, maybe what you want to do is create an issue to look into that later (some of the comments are very much related to cosmetic improvements to generated code, which is good, but probably doesn't need to block this PR).

Feel free to remove the do-not-merge tag if you want to go with the current revision. That should un-block auto-merge.

[mergify]

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: 'BREAKING CHANGE: <describe what changed + link for details>'
  • Issues: Indicate issues fixed via: 'Fixes #xxx' or 'Closes #xxx'