String: add `StringView`, similar to `VecView` #768

Job	Run time
style	12s
clippy	15s
testmiri	1m 6s
check (x86_64-unknown-linux-gnu)	16s
doc (x86_64-unknown-linux-gnu)	21s
check (i686-unknown-linux-musl)	22s
doc (thumbv7m-none-eabi)	26s
check (riscv32imc-unknown-none-elf)	20s
test	21s
check (armv7r-none-eabi)	20s
testcfail	36s
check (thumbv6m-none-eabi)	24s
testcpass (x86_64-unknown-linux-gnu)	11s
check (thumbv7m-none-eabi)	37s
testcpass (x86_64-unknown-linux-gnu, --release)	8s
check (thumbv8m.base-none-eabi)	51s
testcpass (i686-unknown-linux-musl)	21s
check (thumbv8m.main-none-eabi)	17s
testtsan (x86_64-unknown-linux-gnu)	32s
testcpass (i686-unknown-linux-musl, --release)	23s
testtsan (x86_64-unknown-linux-gnu, --release)	41s
	9m 0s

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