Reconcile redis-perf/optim: content superseded by upstream fastfloat#381/fastfloat#382/fastfloat#387

EXP-050/052/053 were upstreamed in final form via fastfloat#381 (integer-scan unroll)
and fastfloat#382 (4-digit follow-up, clang-gated); the fastfloat#387 span-elision restructure
then rewrote the surrounding code. This merge carries the old branch history
while the tree = upstream/main (8.2.7) + EXP-062 (ungate 4-digit SWAR on gcc)
+ EXP-063 (mantissa bound before rounds_to_nearest probe).

NOTE: EXP-052's 2x unroll of loop_parse_if_eight_digits was never upstreamed
and is dropped here pending post-fastfloat#387 revalidation (see EXPERIMENTS.md).

Author: fcostaoliveira