decimal: Add a method to multiply a decimal by an Int.

[ValarDragon]

This is for usage within the slash function, to minimize operations
required to slash each unbonded and redelegating account.

closes #2407

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

---

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 #2408 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop    #2408   +/-   ##
========================================
  Coverage    62.97%   62.97%           
========================================
  Files          121      121           
  Lines         7271     7271           
========================================
  Hits          4579     4579           
  Misses        2374     2374           
  Partials       318      318

[cwgoes]

utACK

[alexanderbez]

Sweet! This will be useful for #2400 -- thanks.