feat: reduce float decoding allocations by using unsafe

[tdakkota]

name                           old time/op    new time/op    delta
Decoder_Float64/floats.json-4    28.6µs ± 3%    23.1µs ± 2%   -19.14%  (p=0.000 n=15+14)

name                           old alloc/op   new alloc/op   delta
Decoder_Float64/floats.json-4    2.39kB ± 0%    0.00kB       -100.00%  (p=0.000 n=15+15)

name                           old allocs/op  new allocs/op  delta
Decoder_Float64/floats.json-4       100 ± 0%         0       -100.00%  (p=0.000 n=15+15)

Fixes #26.

[codecov]

Codecov Report

Merging #39 (9917dfa) into main (2233db5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #39   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           34        35    +1     
  Lines         2304      2309    +5     
=========================================
+ Hits          2304      2309    +5     

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

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