feat(osmomath): BigDec TruncateSDKInt helper

[p0mvn]

Background

Currently, to return an SDK Int from BigDec one must go BigDec -> Dec -> SDK Int

Suggested Design

Introduce a helpers TruncateSDKInt and TruncateSDKIntMut where the latter mutates the BigDec buffer directly and does not do reallocations.

Acceptance Criteria

• 2 helpers implemented
• one does not do reallocations
• each unit tested
• backported to release branch and state-compat confirmed

[p0mvn]

We also need a RountInt helper

[hieuvubk]

NewIntFromBigInt always allocates new big.Int pointer

return Int{new(big.Int).Set(i)}

[hieuvubk]

Waiting for https://github.com/cosmos/cosmos-sdk/pull/18030