Make Anchor consensus-only data (remove txid)

[dr-orlovsky]

This is the alternative to #83 removing non-consensus data from Anchors and achieving the same goal by just delegating SPV storage and verification outside of this library.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 47.7%. Comparing base (7b1ff00) to head (c014bad).

Files	Patch %	Lines
seals/src/txout/witness.rs	0.0%	3 Missing ⚠️
dbc/src/anchor.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #84     +/-   ##
========================================
- Coverage    48.2%   47.7%   -0.5%     
========================================
  Files          36      34      -2     
  Lines        3290    3149    -141     
========================================
- Hits         1587    1503     -84     
+ Misses       1703    1646     -57     

Flag	Coverage Δ
rust	47.7% <60.0%> (-0.5%)	⬇️

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