Design builtin map type with specialized codegen

[grian32]

Goal

Explore a dynamically allocated builtin map type with generated LLVM/codegen specialized per key/value type combination.

Design checklist

• Syntax and type representation for map[K]V or equivalent.
• Hashing rules for primitive keys.
• Equality rules for keys.
• Allocation and ownership model.
• Arena-backed variant or integration.
• Insert/get/remove/contains API.
• Iteration story.
• Codegen strategy for each key/value combination.
• Runtime layout and resizing behavior.

This can start as a design issue before implementation.