* log: fix formatting of big.Int

The implementation of formatLogfmtBigInt had two issues: it crashed when
the number was actually large enough to hit the big integer case, and
modified the big.Int while formatting it.

* log: don't call FormatLogfmtInt64 for int16

* log: separate from decimals back, not front

Co-authored-by: Péter Szilágyi <peterke@gmail.com>