Implement Java codegen for struct-valued constants

[benjamin-bader]

A few minor and not-so-minor changes needed to happen along the way:

• To make this feature bearable, we relaxed the restriction that a struct const must define all fields, and now only required fields must be present.
• This broke Kotlin codegen, so that needed to be fixed
• Turns out that constant-references to enums have always been broken in Java codegen, as revealed by the rather-arbitrary integration test code. The fix for that is sub-optimal but it works.
• When we implemented constant references, we forgot that map keys can be constant references, not just values. Fixed here.

Fixes #484