Remove hasher bound on map and set ser/de

[austinabell]

Backport of changes I needed on Serum's fork https://github.com/project-serum/borsh-rs/pull/1

There is no need for the Hasher to be constrained to the default for serialize/deserialize impls

[ailisp]

Code looks good, however this doesn't build (see ci) when cargo test --verbose --no-default-features, default-feature currently is ["std"]. can you make this change only happen when std?

[austinabell]

Code looks good, however this doesn't build (see ci) when cargo test --verbose --no-default-features, default-feature currently is ["std"]. can you make this change only happen when std?

Oops, I had fixed before but forgot to push

[ailisp]

Looks good, thanks!