blob: add and expose Blob type

[steveej]

Thanks for the input. This seems not worth the risks of exposing the whole module, I'm fine closing it.

[lucab]

I'd say that in the future:

• we may consider if we prefer an opaque newtype for blob (at the moment I don't see the need)
• if we need to expose such a type, we can do selectively only for that (i.e. a pub use instead of a pub mod)