don't iterate over all impls when none match
don't iterate over all impls when none match

before:
573.01user 4.04system 7:33.86elapsed 127%CPU (0avgtext+0avgdata 1141656maxresident)k
after:
567.03user 4.00system 7:28.23elapsed 127%CPU (0avgtext+0avgdata 1133112maxresident)k

an additional 1% improvement

GBGamer/rust,graydon/rust,graydon/rust,dwillmer/rust,graydon/rust,GBGamer/rust,dwillmer/rust,dwillmer/rust,aidancully/rust,graydon/rust,dwillmer/rust,aidancully/rust,GBGamer/rust,dwillmer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,dwillmer/rust,graydon/rust,aidancully/rust,GBGamer/rust,dwillmer/rust,aidancully/rust,dwillmer/rust,GBGamer/rust,aidancully/rust