Add StdError impls for core and alloc errors #4

Job	Run time
Toolchain / stable	14s
Miri /	33s
Toolchain / stable --release	20s
Cross / i686-unknown-linux-gnu	1m 10s
Miri / --release	34s
Toolchain / beta	20s
Clippy	15s
Cross / i586-unknown-linux-gnu	1m 3s
Toolchain / beta --release	14s
Cross / armv7-unknown-linux-gnueabihf	1m 12s
Toolchain / nightly	17s
Doc	15s
Cross / aarch64-unknown-linux-gnu	1m 11s
Features /	15s
Toolchain / nightly --release	18s
Cross / thumbv6m-none-eabi	1m 5s
Features / alloc	14s
Format	12s
Features / std	34s
Test / x86_64-pc-windows-msvc	1m 3s
Test / x86_64-pc-windows-msvc --release	49s
	12m 8s

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