[#790] Straighten out detSecretKey's behavior

NB: I've introduced KeyType instead of reusing HashTag for two reasons:

• Semantically, this seems more correct
• HashTag is a GADT which makes Bounded and Enum instances un-deriveable, and writing those by hand is a huge chore and frankly not worth the effort.

Description

Problem: Morley.Tezos.Crypto.detSecretKey chooses the type of key it generates based on seed length, which can lead to subtle bugs wrt key generation.

Solution: Rewrite it to use seed value instead. Add a couple simple tests. Introduce a variant function detSecretKey' that accepts explicit key type as argument. Introduce KeyType enum for key types.

Problem: KeyType and HashTag duplicate key types between them.

Solution: Replace all key HashTag constructors with a single constructor accepting KeyType as argument.

Related issue(s)

Resolves #790 (closed)

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