Const-evaluate runtime alphabet mapping #330

Job	Run time
Miri	1m 37s
Format, lint, test, and audit	1m 55s
no_std target build (x86_64-unknown-linux-gnu)	17s
Rust compatibility (1.93.0)	28s
no_std target build (thumbv7em-none-eabihf)	17s
Rust compatibility (1.90.0)	28s
no_std target build (x86_64-unknown-freebsd)	19s
Platform tests (macos-15-intel)	1m 2s
Platform tests (macos-26)	1m 20s
no_std target build (wasm32-unknown-unknown)	17s
Platform tests (macos-latest)	43s
no_std target build (aarch64-unknown-linux-gnu)	22s
Platform tests (ubuntu-latest)	34s
Rust compatibility (1.96.1)	27s
Platform tests (macos-15)	36s
Rust compatibility (1.94.0)	23s
Rust compatibility (1.95.0)	25s
Rust compatibility (1.91.0)	31s
Rust compatibility (1.97.0)	15s
Rust compatibility (1.92.0)	26s
Platform tests (windows-latest)	1m 1s
Rust compatibility (1.96.0)	25s
	14m 8s

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