BIP-374: vendor secp256k1lab and use it for reference implementation

[theStack]

Following up on the recent decision that secp256k1lab should be vendored per-BIP if needed/useful (see #2004 (comment) ff. and #1855 (comment) ff.), this PR exercises this for BIP-374. The changes add secp256k1lab version 1.0.0 as subtree [1] within the bip-0374 folder and take use of it in the reference implementation. In particular, secp256k1.py is removed and the TaggedHash and xor_bytes routines are used from the secp256k1lab utils module, to simplify the code further. Usage of the library is mentioned in the BIP text to be license compliant (see #2004 (comment)).

Can be tested via:

$ ./bip-0374/run_test_vectors.py
.....
All test vectors passed.
$ ./bip-0374/gen_test_vectors.py
Generating /home/thestack/bips/bip-0374/test_vectors_generate_proof.csv...
Generating /home/thestack/bips/bip-0374/test_vectors_verify_proof.csv...
$ git diff
<shouldn't show any diffs, as generated .csv files should match the ones in the repository> 

[1] added via the command $ git subtree add --prefix=bip-0374/secp256k1lab --squash https://github.com/secp256k1lab/secp256k1lab v1.0.0

[nymius]

ACK 2b7f079

The changes are aligned with the referenced comments. I'm not reviewing the vendor library as the change is not about the qualities of it.

[macgyver13]

ACK 2b7f079

Tested vector generation and confirmed no changes to generated .csv files.

Additionally tested the same technique in #2046. The next revision of #2046 will depend on the changes in this PR.

[murchandamus]

cc: @andrewtoth, @RubenSomsen; perhaps also of interest to @real-or-random.

[murchandamus]

Also, while I have you here, BIP 374 has been published for over a year, it appears to have a reference implementation and test vectors, do you still have planned work outstanding? Otherwise, you could perhaps consider whether it is time to advance it from Draft to Complete.

[stratospher]

ACK 2b7f079.