Clumsy `Ceremony` implementation for `Config`

[Francisco Hernández Iglesias]

The struct Config in defined manta-trusted-setup::groth16::ceremony::config::ppot as a wrapper over a SignatureScheme is a very clumsy way to implement the Ceremony trait. Probably we should switch from
Ceremony: Configuration + SignatureScheme
to
Ceremony: Configuration { type Signer: SignatureScheme}
to remove the duplicate code.

Originally posted by @bhgomes in #257 (review)