feat: add core-error feature (#32) #101

Job	Run time
clippy	14s
feature checks	15s
docs	15s
fmt	10s
check WASM	6s
test stable --no-default-features	12s
test stable	11s
test stable --all-features	8s
test beta --no-default-features	17s
test beta	14s
test beta --all-features	17s
test nightly --no-default-features	15s
test nightly	17s
test nightly --all-features	14s
test 1.64 --no-default-features	20s
test 1.64	18s
test 1.64 --all-features	14s
	3m 57s

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