Improve code quality in presign imports

[marsella]

There are a few ways in which presign is inconsistent in its naming and module structure compared to the other repos

• lots of renaming of types (e.g. use round_one::Public as RoundOnePublic)
• most of the presign-internal modules are pub(crate) when they could have more limited visibility
• revealing the external-facing presign types happens at lib level instead of in the presign::mod.rs file
• presign reveals the participant module when it doesn't really need to

We should clean this up so that as we start using presign in external settings, the use structure makes sense.

Also, presign's participant module is huge; the Input type can be moved to a separate module. See also: #246