Rename to remove Dbc prefix

[davidrusu]

1. DbcContent -> Content
2. DbcTransaction -> Transaction
3. etc.

Dbc here is superfluous.

[dan-da]

Dbc here is superfluous

Is it? We have other types besides Dbc in the crate. in particular MintNode and it's associated types. All are present in same crate root namespace.

If we changed it so that one could only access via eg: dbc::{Content, Transaction}, then I think I would agree it is superfluous.

[davidrusu]

A dbc module would be good, I'd be down with that