Fix an inference failure (fn found, different fn expected.)
Fix an inference failure (fn found, different fn expected.)

For some reason the inference is tagging the `fn()` for the key-setup function
with the name of the setup function. So the type inferred for the expression can only
be satisfied by the first match-arm.

Adding a type-hint seems to resolve this issue.

nmathewson/rust-crypto,mseri/rust-crypto,thinkingfish/rust-crypto,PeterReid/rust-crypto,tempbottle/rust-crypto,PeterReid/rust-crypto,canndrew/rust-crypto,vladikoff/rust-crypto,andreasots/rust-crypto,andreasots/rust-crypto,DaGenix/rust-crypto,mseri/rust-crypto,canndrew/rust-crypto,thinkingfish/rust-crypto,tempbottle/rust-crypto,exe-dealer/rust-md5,DaGenix/rust-crypto,nmathewson/rust-crypto