Impl Borrow and BorrowMut for String and Vec. #856

Job	Run time
style	10s
check (x86_64-unknown-linux-gnu)	16s
test	23s
check (i686-unknown-linux-musl)	22s
check (riscv32imc-unknown-none-elf)	19s
clippy	13s
check (armv7r-none-eabi)	26s
check (thumbv6m-none-eabi)	24s
doc (x86_64-unknown-linux-gnu)	20s
testcfail	21s
doc (thumbv7m-none-eabi)	24s
check (thumbv7m-none-eabi)	16s
check (thumbv8m.base-none-eabi)	26s
check (thumbv8m.main-none-eabi)	18s
testcpass (x86_64-unknown-linux-gnu)	12s
testmiri	1m 3s
testcpass (x86_64-unknown-linux-gnu, --release)	20s
testcpass (i686-unknown-linux-musl)	19s
testtsan (x86_64-unknown-linux-gnu)	35s
testcpass (i686-unknown-linux-musl, --release)	11s
testtsan (x86_64-unknown-linux-gnu, --release)	40s
	7m 58s

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