Add component_sub_point to composer

[xevisalle]

Resolves: #842

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.55%. Comparing base (0e75be5) to head (e79633d).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/ecc.rs	92.15%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
+ Coverage   85.44%   85.55%   +0.10%     
==========================================
  Files          57       57              
  Lines        3725     3781      +56     
==========================================
+ Hits         3183     3235      +52     
- Misses        542      546       +4     

Files with missing lines	Coverage Δ
src/composer.rs	90.55% <100.00%> (+0.18%)	⬆️
tests/ecc.rs	89.23% <92.15%> (+1.88%)	⬆️

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[moCello]

General LGTM, although while we're at it, I would also add a component_neg_point.

[marta-belles]

LGTM