Add arena std module and arena-allocating variants

[grian32]

Goal

Introduce an arena allocation std module and arena-taking variants of allocation-heavy stdlib APIs.

Tasks

• Design arena type/API: create, alloc, reset, free.
• Decide ownership/lifetime conventions.
• Add arena variants for strings.
• Add arena variants for arrays.
• Ensure future map/container APIs can use arenas.
• Document examples and failure behavior.

Refs: STDLIB.md, builtins/strings.gl3, builtins/arrays.c.