Merge `Signer`/`Verifier`/`Signature`/`Digest` into `SessionParameters` #31

Job	Run time
clippy	28s
coveralls	41s
rustfmt	14s
build (1.81.0, wasm32-unknown-unknown)	27s
build (stable, wasm32-unknown-unknown)	26s
semver	24s
tests (x86_64-unknown-linux-gnu, stable)	37s
	3m 17s

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