Add skip_verification() and to_unverified() to kfrags/cfrags

[fjarri]

Fixes #73

• adds KeyFrag::skip_verification()
• adds VerifiedKeyFrag::to_unverified()
• adds CapsuleFrag::skip_verification()
• adds VerifiedCapsuleFrag::to_unverified()
• adds corresponding methods to Python and WASM bindings

[codecov-commenter]

Codecov Report

Merging #84 (3fbc0b6) into master (22645d1) will increase coverage by 0.02%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   81.12%   81.15%   +0.02%     
==========================================
  Files          15       15              
  Lines        1118     1125       +7     
==========================================
+ Hits          907      913       +6     
- Misses        211      212       +1     

Impacted Files	Coverage Δ
umbral-pre/bench/bench.rs	0.00% <0.00%> (ø)
umbral-pre/src/bench.rs	0.00% <ø> (ø)
umbral-pre/src/capsule_frag.rs	89.58% <66.66%> (-0.49%)	⬇️
umbral-pre/src/key_frag.rs	90.55% <66.66%> (-0.41%)	⬇️
umbral-pre/src/capsule.rs	90.83% <100.00%> (+0.07%)	⬆️
umbral-pre/src/pre.rs	92.30% <100.00%> (+0.12%)	⬆️

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