Fix `shadowed lifetimes` warnings ...
Fix `shadowed lifetimes` warnings ...

Lifetime `'a` declared on functions is shadowing the lifetime `'a` decl.
by the struct.

However all these lifetimes can be inferred by lifetime ellision, so
they can be eliminated entirely.

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