R4R: Make StdSignature turn into interface

[alessio]

Ref #3198

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3283 into develop will increase coverage by 0.06%.
The diff coverage is 84.21%.

@@             Coverage Diff             @@
##           develop    #3283      +/-   ##
===========================================
+ Coverage    55.18%   55.25%   +0.06%     
===========================================
  Files          134      134              
  Lines         9526     9531       +5     
===========================================
+ Hits          5257     5266       +9     
+ Misses        3938     3934       -4     
  Partials       331      331

[ValarDragon]

RIP 4 more bytes in every tx

[sunnya97]

Hmm, upon looking at this, I'm actually reconsidering. When I proposed this idea, I didn't realize that

1. The method for verifying a signature is pubKey.VerifyBytes(signBytes []byte, sigatureBytes []byte) rather than a method on the StdSignature itself.

2. And also I didn't realize that the multisig package already has a multisignature type.

I didn't realize that a lot of this functionality was already implemented by the multisig package. Given this, I'm not sure that the interface is necessary. Thoughts?

[alessio]

I'm fine with that @sunnya97