Follow up to #2686 -- Coins API

[alexanderbez]

Summary

I believe the changes introduced in #2686 may lead to a confusing API to most users. Take for example:

assert.True(t, Coins{{"A", one}, {"B", one}}.IsAllGT(Coins{{"B", one}}))

The coin A is not in the "other" set, but based off of the godoc, it would make you seem to believe this should return false.

We should at the very least update the godocs.

/cc @jaekwon @alessio

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[alessio]

IMHO we should remove all comparison ops from the type Coins and encourage the use of AmountOf + Coin's comparison ops.

[alexanderbez]

@alessio this is what we do mostly in x/bank I believe. But yes, good point. The API is still confusing though.

[jackzampolin]

This code has undergone significant refactor by @alessio since this issue. Going to go ahead and close.