elements: support explicit transaction signing

[romanz]

Implements initial support for Elements' transaction signatures (following #66).

Most of the code is similar to Bitcoin's, but there are a few deviations:

• amount serialization is different (to support explicit & confidential assets).
• segwit is enabled for all transactions and witness marker is not used.
• BIP-143 serialization is a bit different (new issuance field and different amount type).
• nLockTime is serialized before the witness.
• there are more input-related witnesses (issuance, re-issuance & peg-in).
• there are output-related witnesses (range-proof & surjection-proof).

In order to make the review easier, this PR adds supports only the explicit assets & values, without blinding/unblinding (depends on #317), peg-in & issuance support.

TODOs:

• the UI is currently doesn't show the asset type being transacted.
• uint64 amount and TxConfidentialValue confidential_value are not verified for equivalence.

[romanz]

We have a PoC for signing Elements transactions (both explicit & confidential) at elements/wip branch, whose device tests work well on the emulator and the TREZOR model T device:

$ pipenv shell
$ pip install install 'git+https://github.com/romanz/python-elementstx@trezor'  # for Elements' transactions
$ PYOPT=0 make -C core clean build_unix build_firmware

$ (cd core; build/unix/micropython src/main.py &)  # Load emulator
$ py.test tests/device_tests/ -vvxsk elements

$ trezorctl firmware-update -f core/build/firmware/firmware.bin  # Load device
$ sudo cat /dev/ttyACM0 &
$ py.test tests/device_tests/ -vvxsk elements

Should I close this PR and open a new one?

[tsusanka]

Hi there, thank you for your PR. I just wanted to give you a quick update from our side. We are interested in adding this to our firmware but we just don't have enough capacity to review it right now. We would be more than happy to take a closer look at it later, when our capacity frees up a little, likely during the course of 2021. Thank you for your understanding.