`ArrayString`: Reducing reliance on `unsafe` blocks + adding some safety comments #224

Job	Run time
nostd/thumbv6m-none-eabi/stable	13s
tests (1.51.0, serde, false, --skip spare_capacity_mut)	2m 35s
miri	44s
tests (1.70.0, serde, false)	31s
tests (stable, true, false)	24s
tests (beta, serde, false)	23s
tests (nightly, serde, borsh, zeroize, false)	22s
	5m 12s

Provide feedback