Tests

nit: remove unnecessary call to into_u256() for bn128 add (#1239) #2

Job	Run time
check serde	24s
check no-default-features	28s
check std	27s
clippy	1m 14s
docs	1m 24s
fmt	15s
test stable --no-default-features	1m 26s
test no_std	40s
test stable	59s
test stable --all-features	1m 16s
test beta --no-default-features	57s
test beta	52s
test beta --all-features	1m 13s
test nightly --no-default-features	1m 4s
test nightly	1m 6s
test nightly --all-features	1m 16s
	15m 1s

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