Murisi/box impl build params

[murisi]

Implemented some changes to facilitate the signing of MASP Transactions using the hardware wallet:

• Implemented BuildParams for boxes to enable consumers to dynamically switch between RngBuildParams and StoredBuildParams
• Make authorization mappers provide positional information to allow bundle signatures to be replaced with an ordered list (coming from the hardware wallet)
• Made a new key type that explicitly stores ask, ak, nsk, and nk to allow consumers to force transaction construction in the presence of inconsistencies/incomplete information (like known ak but unknown ask)
• Generalized transaction Builders to allow key implementations other than ExtendedSpendingKey which might not enforce certain relations to be used

[tzemanovic]

LGTM (modulo fmt). The MaybeArbitrary is also a lot cleaner than before, thanks for refactoring it!