Problem: multiple denoms can be mapped to same contract

[yihuang]

Closes: #186

Solution:

• add validation that the contract is not already mapped.

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

PR Checklist:

• Have you read the CONTRIBUTING.md?
• Does your PR follow the C4 patch requirements?
• Have you rebased your work on top of the latest master?
• Have you checked your code compiles? (make)
• Have you included tests for any non-trivial functionality?
• Have you checked your code passes the unit tests? (make test)
• Have you checked your code formatting is correct? (go fmt)
• Have you checked your basic code style is fine? (golangci-lint run)
• If you added any dependencies, have you checked they do not contain any known vulnerabilities? (go list -json -m all | nancy sleuth)
• If your changes affect the client infrastructure, have you run the integration test?
• If your changes affect public APIs, does your PR follow the C4 evolution of public contracts?
• If your code changes public APIs, have you incremented the crate version numbers and documented your changes in the CHANGELOG.md?
• If you are contributing for the first time, please read the agreement in CONTRIBUTING.md now and add a comment to this pull request stating that your PR is in accordance with the Developer's Certificate of Origin.

Thank you for your code, it's appreciated! :)

[codecov]

Codecov Report

Merging #187 (6aac78e) into main (3ea70c5) will increase coverage by 5.21%.
The diff coverage is 54.07%.

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   21.51%   26.73%   +5.21%     
==========================================
  Files          27       33       +6     
  Lines        1729     2338     +609     
==========================================
+ Hits          372      625     +253     
- Misses       1324     1666     +342     
- Partials       33       47      +14     

Impacted Files	Coverage Δ
app/prefix.go	0.00% <0.00%> (ø)
app/test_helpers.go	0.00% <0.00%> (ø)
x/cronos/keeper/grpc_query.go	0.00% <0.00%> (ø)
x/cronos/keeper/msg_server.go	4.76% <0.00%> (-1.69%)	⬇️
x/cronos/module.go	59.64% <0.00%> (-2.17%)	⬇️
x/cronos/types/codec.go	0.00% <0.00%> (ø)
x/cronos/types/events.go	0.00% <ø> (ø)
x/cronos/types/messages.go	20.22% <ø> (+20.22%)	⬆️
x/cronos/types/params.go	57.35% <ø> (+3.78%)	⬆️
x/cronos/types/proposal.go	11.11% <ø> (ø)
... and 20 more

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[thomas-nguy]

lgtm