R4R: improve denom validation

[alessio]

• Use a more precise regular expression to match denom strings (e.g. [[:lower:]] instead of [[:alpha:]]) and avoid trimming spaces.
• validateDenom() -> validateDenom() error, use mustValidateDenom() in constructors to forcibly panic when validation fails.

• 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)

[cwgoes]

Concept ACK, we should do this sooner rather than later since it reduces the space of acceptable denoms.

[codecov]

Codecov Report

Merging #3666 into develop will decrease coverage by <.01%.
The diff coverage is 78.94%.

@@             Coverage Diff             @@
##           develop    #3666      +/-   ##
===========================================
- Coverage    61.39%   61.39%   -0.01%     
===========================================
  Files          189      189              
  Lines        14098    14100       +2     
===========================================
  Hits          8656     8656              
- Misses        4886     4888       +2     
  Partials       556      556

[jackzampolin]

I like this. Also agree with @cwgoes

[okwme]

Was the limit to 3 characters something that existed before this PR? Was there more reason than limiting the space of acceptable denoms? I'm trying to build an app with single char denoms rn and wondering if there's a good reason not to (besides forking the SDK 🍴)

cc @ebuchman

[ebuchman]

Goes back even further: 2c1d533#diff-641f1d674d28f26344e36b61db7f47beL12

Probably it's to avoid people squatting smaller denoms and issues with readability/differentiation with less than 3 letters