[issue1580] Use shorter float repr when possible

[Tim Peters]

Tim Peters added the comment:

Again, without replacing float input routines too, this is /not/ good
enough to ensure eval(repr(x)) == x even across just 100%-conforming 754
platforms.

It is good enough to ensure (well, assuming the code is 100% correct)
eval(repr(x)) == x across conforming 754 platforms that go /beyond/ the
standard by also supplying correctly rounding input routines.  Some
platform C libraries do, some don't.  For example, I believe glibc does
(and builds its I/O code on David Gay's, mentioned before), but that
Microsoft's does not (but that Microsoft's do meet the 754 standard,
which-- again --isn't strong enough for "shortest output" to round-trip
correctly in all cases).

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