Fix RBS translation of braceless shape return types

[Morriar]

Fixes #641

• preserve braceless hash-rocket return shapes by restoring braces around Prism keyword hash nodes
• distinguish hash rockets from keyword arguments using association operator locations
• cover string, symbol, mixed-key, single-key, and multi-key shapes
• keep invalid keyword-argument forms rejected during RBS translation

[Morriar]

No comments from me. The parser change is small and covered by parser/RBS printer tests.