Fix BN.js type hacks

[zone117x]

Description

Closes #751

Fixes the BN.js type problems -- nows that the @types/bn.js package has been updated with DefinitelyTyped/DefinitelyTyped#41255

Downstream typescript dependencies no longer require skipLibCheck

Type of Change

• New feature
• Bug fix
• API reference/documentation update
• Other

Checklist

• Code is commented where needed
• Unit test coverage for new or modified code paths
• npm run test passes
• Changelog is updated
• Tag 1 of @yknl or @zone117x for review

[codecov]

Codecov Report

Merging #753 into feature/content-length-checking will increase coverage by 0.02%.
The diff coverage is 88.46%.

@@                         Coverage Diff                         @@
##           feature/content-length-checking     #753      +/-   ##
===================================================================
+ Coverage                            68.72%   68.75%   +0.02%     
===================================================================
  Files                                   65       64       -1     
  Lines                                 3814     3808       -6     
  Branches                               692      693       +1     
===================================================================
- Hits                                  2621     2618       -3     
+ Misses                                 867      863       -4     
- Partials                               326      327       +1

Impacted Files	Coverage Δ
src/operations/txbuild.ts	89.82% <ø> (ø)	⬆️
src/network.ts	75.06% <100%> (ø)	⬆️
src/operations/utils.ts	95.5% <100%> (ø)	⬆️
src/operations/skeletons.ts	79.48% <50%> (ø)	⬆️
src/encryption/ec.ts	84.67% <83.33%> (-3.56%)	⬇️

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[yknl]

Looks good, thanks for the bn.js PR in support of this too.