[stdlib] Minor clean-up and new fast path for larger (e.g. 128-bit) integer printing (#85404)
A follow-up to #85180.
First, make the guarantee that our Swift-native implementation of
integer-to-ASCII conversion always fills the _suffix_ of the given
mutable span.
(Some minor swift-format corrections interspersed.)
Then, use that guarantee to work in 64-bit (or rather, for most bases,
56-bit) chunks for larger integers.
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7d3b2576 - [stdlib] Minor clean-up and new fast path for larger (e.g. 128-bit) integer printing (#85404)
