Add `alloc` feature with `Box`/`Vec` conversions #280

Job	Run time
clippy	18s
build (stable, armv7a-none-eabi)	18s
careful	57s
build (1.85.0, wasm32-unknown-unknown)	20s
miri (x86_64-unknown-linux-gnu)	43s
build (1.85.0, thumbv7em-none-eabi)	21s
doc	12s
rustfmt	10s
build (stable, wasm32-unknown-unknown)	21s
test (1.85.0)	22s
miri (s390x-unknown-linux-gnu)	52s
test (stable)	36s
build (1.85.0, armv7a-none-eabi)	20s
build (stable, thumbv7em-none-eabi)	20s
	6m 10s

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