apd: add inline fast-path to BigInt.Format
apd: add inline fast-path to BigInt.Format

This commit adds an inline fast-path to (*BigInt).Format. This method is used in
CockroachDB's `encoding.decimalEandM` function, so it's worth optimizing.
Without the fast-path, (*big.Int).Format performs at least one heap allocation
to store a temp value.

The commit also removes a few calls to `(*BigInt).innerOrNil` in performance
sensitive methods, because this function prevents `(*BigInt).inner` from being
inlined.

cockroachdb/apd