Add type-safe `VecMap` for slices indexed by newtypes #454

Job	Run time
Configure	3s
Integration Neko	7m 8s
Miri / Miri	8m 51s
QPY / Test	9m 5s
Image / Test-images-3.10	4m 6s
Lint / Lint	5m 23s
Documentation / Docs	12m 45s
Unit Rust / ubuntu-latest	8m 3s
Unit Python / Windows / Python newest / windows-latest - Python 3.14	24m 36s
Unit Python / Windows / Python oldest / windows-latest - Python 3.10	23m 10s
Unit Python / macOS / Python newest / macos-15 - Python 3.14	11m 58s
Unit Python / Linux / Python oldest - ubuntu-24.04-arm / ubuntu-24.04-arm - Python 3.10	13m 19s
Unit Python / Linux / Python oldest - ubuntu-latest / ubuntu-latest - Python 3.10	18m 7s
Unit Python / macOS / Python oldest / macos-15 - Python 3.10	11m 1s
Unit Python / Linux / Python newest - ubuntu-24.04-arm / ubuntu-24.04-arm - Python 3.14	8m 49s
Unit C / ubuntu-latest / ubuntu-latest	2m 39s
Unit Python / Linux / Python newest - ubuntu-latest / ubuntu-latest - Python 3.14	10m 56s
Unit C / ubuntu-24.04-arm / ubuntu-24.04-arm	3m 23s
Unit C / macos-15 / macos-15	2m 11s
Unit C / windows-latest / windows-latest	5m 8s
Finalize	3s
	3h 10m 44s

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