feat: improve float decoding layout, add more float benchmarks

[tdakkota]

name                                old time/op    new time/op    delta
Decoder_Float64/floats.json-4         3.17µs ± 4%    2.96µs ± 1%   -6.66%  (p=0.000 n=23+25)
Decoder_Float64/slow_floats.json-4    21.2µs ± 2%    20.8µs ± 2%   -1.93%  (p=0.000 n=23+25)
Decoder_Float64/integers.json-4       4.32µs ± 2%    3.85µs ± 4%  -10.92%  (p=0.000 n=24+25)

[codecov]

Codecov Report

Merging #41 (9f27e28) into main (e09ad96) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #41   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines         2307      2307           
=========================================
  Hits          2307      2307           

Impacted Files	Coverage Δ
dec_float.go	100.00% <100.00%> (ø)

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