feat(mint): add mint module

[giunatale]

Closes #780
Closes #781

What does this PR does?

Adds a forked version of the mint module.
Unfortunately I could not fork the osmosis module since it depends on their epoch module, and so it was unfit.
I opted to start with the vanilla mint module as a baseline, with the dynamic inflation rate. In the future, we could explore some capping mechanism, for instance a reduction after a year worth of blocks as done in the juno mint module.

The module distributes inflation to 3 parties:

• staking
• the future-to-be module account of the incentive modules (currently commented out in keeper.go)
  the idea is to send rewards directly to the incentives module account so that the module itself can handle the accumulated rewards independently.
• community pool

I have opted to not include at this time a "developers" rewards or "strategic reserve" rewards because this need to be thought out later. We could explore a similar design as per osmosis where a weighted list of addresses, that can be changed through governance, receive rewards from inflation. Not sure if doing this per-block is efficient though. Hence, leaving it aside.

[codecov]

Codecov Report

Merging #794 (381e6b3) into develop (af6dccb) will decrease coverage by 0.08%.
The diff coverage is 8.28%.

@@             Coverage Diff             @@
##           develop     #794      +/-   ##
===========================================
- Coverage    10.73%   10.65%   -0.09%     
===========================================
  Files          286      302      +16     
  Lines        69593    72212    +2619     
===========================================
+ Hits          7473     7693     +220     
- Misses       61940    64333    +2393     
- Partials       180      186       +6     

Impacted Files	Coverage Δ
x/mint/abci.go	0.00% <0.00%> (ø)
x/mint/genesis.go	0.00% <0.00%> (ø)
x/mint/types/genesis.go	0.00% <0.00%> (ø)
x/mint/types/query.pb.gw.go	0.00% <0.00%> (ø)
x/mint/types/mint.pb.go	0.57% <0.57%> (ø)
x/mint/types/query.pb.go	0.94% <0.94%> (ø)
x/mint/types/genesis.pb.go	1.10% <1.10%> (ø)
x/mint/types/params.go	9.03% <9.03%> (ø)
x/mint/client/rest/query.go	31.57% <31.57%> (ø)
x/mint/keeper/querier.go	74.28% <74.28%> (ø)
... and 7 more

[lumtis]

LGTM staking rewards can be distributed

I would suggest setting params for the module in config.yml

[aljo242]

This and a minor lint issue need to be fixed imo. I can just push these minor changes.