strconv: implement parseFloatPrefix returning no. of bytes consumed

strconv: implement parseFloatPrefix returning no. of bytes consumed

parseFloatPrefix will make it easier to implement ParseComplex.

Verified that there's no relevant performance impact:
Benchmarks run on a "quiet" MacBook Pro, 3.3GHz Dual-Core Intel Core i7,
with 16GB 2133MHz LPDDR3 RAM running macOS 10.15.4.

name                  old time/op  new time/op  delta
Atof64Decimal-4       38.2ns ± 4%  38.4ns ± 3%    ~     (p=0.802 n=5+5)
Atof64Float-4         41.1ns ± 3%  43.0ns ± 1%  +4.77%  (p=0.008 n=5+5)
Atof64FloatExp-4      71.9ns ± 3%  70.1ns ± 1%    ~     (p=0.063 n=5+5)
Atof64Big-4            124ns ± 5%   119ns ± 0%    ~     (p=0.143 n=5+4)
Atof64RandomBits-4    57.2ns ± 1%  55.7ns ± 2%  -2.66%  (p=0.016 n=4+5)
Atof64RandomFloats-4  56.8ns ± 1%  56.9ns ± 4%    ~     (p=0.556 n=4+5)
Atof32Decimal-4       35.4ns ± 5%  35.9ns ± 0%    ~     (p=0.127 n=5+5)
Atof32Float-4         39.6ns ± 7%  40.3ns ± 1%    ~     (p=0.135 n=5+5)
Atof32FloatExp-4      73.7ns ± 7%  71.9ns ± 0%    ~     (p=0.175 n=5+4)
Atof32Random-4         103ns ± 6%    98ns ± 2%  -5.03%  (p=0.008 n=5+5)

Updates #36771.

Change-Id: I8ff66b582ae8b468d89c9ffc35c569c735cf0341
Reviewed-on: https://go-review.googlesource.com/c/go/+/230737
Reviewed-by: Ian Lance Taylor <iant@golang.org>