Move code from elements-fun into rust-elements

[thomaseizinger]

It turns out that there isn't as much need for most of the re-modelling that we did in elements-fun as we initially thought. See more here.

This is a tracking issue for moving all of what we did in elements-fun back into rust-elements (non-exhaustive list):

• Add a dependency to rust-secp256k1-zkp and model asset/value commitment using types from rust-secp256k1-zkp
• Create TxOut constructors for new_last_confidential / new_non_last_confidential
• Introduce .unblind API on TxOut
• Introduce ExplicitTxOut and ConfidentialTxOut along with into_{explicit,confidential} on TxOut

[luckysori]

This is so done.