[#456] Replace lSignEd22519 with generic lSign

Description

Problem: morley-multisig tests are using lSignEd22519 which uses low-level primitives from Tezos.Crypto.Ed25519. For the sake of uniformity, it is better to have a general signing method working with primitives from Tezos.Crypto.

Solution: (1) Replace lSignEd22519 with a more generic lSign based on primitives from Tezos.Crypto; (2) Update morley-multisig to use new lSign.

This PR only deals with (1) on account of morley-multisig being in a separate repo.

(2) is tracked in morley-multisig!17 (merged)

Notice we need to add an explicit dependency on cryptonite to be able to write the type of the new lSign function, since Tezos.Crypto primitives require the MonadRandom constraint.

Related issue(s)

Related to #456 (closed).

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• Tests (see short guidelines)
  • If I added new functionality, I added tests covering it.
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There are seemingly no tests in the lorentz subproject. I think adding tests from scratch is out of scope for this MR.

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