Merge `Signer`/`Verifier`/`Signature`/`Digest` into `SessionParameters` #28

Job	Run time
clippy	27s
coveralls	40s
rustfmt	12s
build (1.81.0, wasm32-unknown-unknown)	27s
build (stable, wasm32-unknown-unknown)	26s
semver	21s
tests (x86_64-unknown-linux-gnu, stable)	35s
	3m 8s

Provide feedback