Consistent use of Plus vs Add etc.

[rigelrozanski]

Relevant for Coins, DecCoins, Decimal, Int etc. libraries in the SDK

Possible inconsistencies to verify:
Plus vs. Add
Minus vs. Sub
Div vs. Quo
Times vs. Mul

My preference is the latter of each of these v/s

[alexanderbez]

++

[rigelrozanski]

As a part of this I think we should make consistent MulRaw and MulInt64 between Int and Dec again the latter is my preference