Feat: new circuits part 1

[SupremoUGH]

First chunk of #153. It consists of the changes in the abstractions in manta-util and manta-crypto.

Changes:
manta-util:

• codec: Encode and Decode implementations for Box.
• num: CheckedIncrement, CheckedDecrement and impl_checked macro.

manta-crypto:

• algebra: new diffie_hellman module.
• constraint: Satisfied trait.
• encryption-convert: new header module.
• encryption: unsafe encryption added.
• key: expanded key agreement to include EphemeralSecretKey and EphemeralPublicKey. The modules agreement and kdf are in two different files now.
• permutation-duplexer: Duplexer implements Encode and Decode.

manta-accounting and manta-pay: minimal changes so the changes above compile.

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