Add dex module skeleton.

[dzmitryhil]

Description

Add dex module skeleton.

Reviewers checklist:

• Try to write more meaningful comments with clear actions to be taken.
• Nit-picking should be unblocking. Focus on core issues.

Authors checklist

• Provide a concise and meaningful description
• Review the code yourself first, before making the PR.
• Annotate your PR in places that require explanation.
• Think and try to split the PR to smaller PR if it is big.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.09%. Comparing base (f0664d2) to head (f60de55).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
- Coverage   36.79%   36.09%   -0.70%     
==========================================
  Files         165      180      +15     
  Lines       49342    50627    +1285     
==========================================
+ Hits        18156    18276     +120     
- Misses      27742    28902    +1160     
- Partials     3444     3449       +5     

Flag	Coverage Δ
coreum	31.74% <ø> (-0.63%)	⬇️
coreum-integration-tests-modules	22.54% <ø> (-0.39%)	⬇️

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[masihyeganeh]

Reviewable status: 0 of 27 files reviewed, all discussions resolved (waiting on @miladz68, @wojtek-coreum, and @ysv)

[ysv]

Reviewed 27 of 27 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @miladz68 and @wojtek-coreum)

[dzmitryhil]

Reviewed 23 of 27 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @masihyeganeh, @miladz68, @wojtek-coreum, and @ysv)

[masihyeganeh]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @miladz68, @wojtek-coreum, and @ysv)

[miladz68]

Reviewed 23 of 27 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wojtek-coreum and @ysv)