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Add Eisel-Lemire decimal parsing fast path (#3957)

* Add Eisel-Lemire decimal parsing fast path

* Fix Eisel-Lemire license headers

* test(strconv,json): quickcheck properties for the Eisel-Lemire fast path

Whenever try_eisel_lemire64 accepts an input it must return the correctly
rounded double, so both new strconv properties compare accepted results
against the exact Decimal conversion, reached through parse_double by
zero-padding the mantissa spelling past the 19-digit fast-path limit. One
draws mantissas across every magnitude; the other clusters them around
powers of two where rounding carries and halfway ambiguity live.

The json property pins the scanner wiring: plain, fraction-padded, and
pointed spellings of the same value must parse to the same bits, with the
padded spelling forcing the exact fallback that anchors the other routes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Hongbo Zhang <bobzhang1988@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

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