Do not keep explicit data on seal close method in DBC anchor

[dr-orlovsky]

The anchor contains an explicit DBC proof, which commits to the seal closing method by its type.

This PR also relaxes 1-to-1 match between DBC data type and closing method: before a DBC type was usable only for a single close method (since it dbc::Proof had a METHOD as a const). Now, enumeration DBC types, with different closing methods for each of the variants are supported. This API change was required for fixing a bug downstream in RGB crates prevending from using operations involving multiple seal seals with different closing methods simultaneously.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 42.2%. Comparing base (f2bbab6) to head (ec9b612).

Files with missing lines	Patch %	Lines
dbc/src/anchor.rs	0.0%	4 Missing ⚠️
dbc/src/opret/mod.rs	0.0%	1 Missing ⚠️
dbc/src/tapret/mod.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #105   +/-   ##
======================================
  Coverage    42.2%   42.2%           
======================================
  Files          35      35           
  Lines        3514    3511    -3     
======================================
  Hits         1483    1483           
+ Misses       2031    2028    -3     

Flag	Coverage Δ
rust	42.2% <0.0%> (+<0.1%)	⬆️

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