refactor: Move sdk.Dec to math package

[amaury1093]

Description

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[julienrbrt]

Can we add this same entry in the root changelog as well?

[faddat]

massively appreciated change.

Thanks @AmauryM !

[aaronc]

Can we call this math.LegacyDec please? (Keeping aliases as sdk.Dec is still fine). And also start merging the GDA/apd code so we get that done for once and for all (ref #11783)?

The code I'm referring to is this code: https://github.com/regen-network/regen-ledger/blob/master/types/math/dec.go. We've had it tested and in use for a while - the main difference is this Dec returns errors rather than panicking which I think is good.

[amaury1093]

I did this in 7304e08. TBH I'm not convinced. The API looks ugly with Legacy in front of all the names.

One advantage of having versioned go modules is that we could stop calling stuff legacy, right? I'm more in favor or keeping it in v1. sdk.Dec, even though it has bugs, has been used long enough in production chains to be deemed as v1.