Use dedicated error types for Add and Not derives (#221)

[tyranron]

Part of #221

Synopsis

See #221 (comment)

Solution

Adds ops::UnitError, ops::WrongVariantError and ops::BinaryError to derive_more crate and uses them in Add-like and Not-like derives.

Additionally:

• Renames derive_more::errors module as derive_more::convert to follow std/core naming conventions.
• Adds #[doc(hidden)] attribute on to new() methods of created error types. This way we mimic them as not public, thus won't introduce breaking changes in future once we need to modify their private fields.

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[JelteF]

Thanks! looks good.