Optimize Int to String (Display) methods.

[arthurprs]

As of today rust uses a naive implementation for converting integers to decimal strings in _Int_ Debug/Display methods.

I propose adding an optimization for the most commonly used case, that is, printing as decimals.

I wrote a reasonably optimized version here. Further optimizations are possible but I tried to keep the code size small (which I think is important), it's a road of diminished gains.

This should be non measurable overall but might give us a minor speedup on _serde_ and _rustc_ serializers.

Should I adapt the code and file PR?

[steveklabnik]

Should I adapt the code and file PR?

I'd want something more than 'might give us a minor speedup', if you can post benchmarks that show it's faster, then we'd probably take such a thing, but benchmarks are important :)

(also, https://internals.rust-lang.org/ might be a better forum for this kind of question, as it isn't really a bug, exactly. So closing!)

[arthurprs]

Follow up thread on internals, including benchmarks https://internals.rust-lang.org/t/optimize-int-to-string-conversions/2245