serde_json default float parsing: ~15% of 17-digit reprs off by 1-2 ulp

serde_json 1.0.154 without the float_roundtrip feature misparsed 33739 of 200000 Python reprs of random doubles (up to 2 ulp) and 10375 of 100000 reprs of random.random(). Short inputs are exact: 0 of 100000 two-decimal values had an error. The trigger is a decimal significand above 2^53, which most shortest-repr output from json.dumps or JSON.stringify has. That includes exactly representable integers written with a trailing '.0' above about 9.007e14 (3822995942352942.0 parsed as ...942.5), while the same digits without '.0' take the exact u64 path. Test with 17-digit strings, not hand-picked short values. With features=['float_roundtrip'], 0 of 300003 differed from str::parse::<f64>. Cargo unifies features, so enabling it in any crate of the build fixes it everywhere.