UnmarshalJSON, Scan and NewFromString performance improvements

[PauliusLozys]

This PR improves UnmarshalJSON performance by reducing unnecessary allocation caused by unquoteIfQuoted function. This also touches on Scan method to split default case into string and []byte cases.

This PR also slightly touches NewFromString function by making so scientific notation and dots are checked in a single loop. Also, adds an additional check (and tests) for multiple scientific notations in the string.

Benchmarks:
Old:

goos: linux
goarch: amd64
pkg: github.com/shopspring/decimal
cpu: AMD Ryzen 7 7700X 8-Core Processor             
BenchmarkDecimal_UnmarshalJSON-16    	10973582	       105.5 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11399134	       103.7 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11480460	       103.4 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11440952	       103.6 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11305298	       103.2 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11353047	       102.5 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11419255	       103.4 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11392092	       103.3 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11341843	       104.6 ns/op	      96 B/op	       5 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	11605540	       103.0 ns/op	      96 B/op	       5 allocs/op
PASS
ok  	github.com/shopspring/decimal	12.861s

New:

goos: linux
goarch: amd64
pkg: github.com/shopspring/decimal
cpu: AMD Ryzen 7 7700X 8-Core Processor             
BenchmarkDecimal_UnmarshalJSON-16    	13650522	        83.84 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14697308	        80.16 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14545791	        80.81 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14889534	        79.66 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	15066388	        79.93 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14768164	        80.08 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14964182	        80.74 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	15048375	        80.77 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14638641	        80.14 ns/op	      72 B/op	       4 allocs/op
BenchmarkDecimal_UnmarshalJSON-16    	14878336	        80.23 ns/op	      72 B/op	       4 allocs/op
PASS
ok  	github.com/shopspring/decimal	12.701s

Benchstat comparison:

goos: linux
goarch: amd64
pkg: github.com/shopspring/decimal
cpu: AMD Ryzen 7 7700X 8-Core Processor             
                         │   old.txt    │               new.txt               │
                         │    sec/op    │   sec/op     vs base                │
Decimal_UnmarshalJSON-16   103.40n ± 1%   80.20n ± 1%  -22.44% (p=0.000 n=10)

                         │  old.txt   │              new.txt               │
                         │    B/op    │    B/op     vs base                │
Decimal_UnmarshalJSON-16   96.00 ± 0%   72.00 ± 0%  -25.00% (p=0.000 n=10)

                         │  old.txt   │              new.txt               │
                         │ allocs/op  │ allocs/op   vs base                │
Decimal_UnmarshalJSON-16   5.000 ± 0%   4.000 ± 0%  -20.00% (p=0.000 n=10)

[mwoss]

Overall looks good! Thanks for the contribution!

[mwoss]

Just for the sake of consistency, can we do a similar check as below? I mean, check for e or E and if value already set, return an error. What do you think?

[PauliusLozys]

Hi, yes, no problem 👍. Added the check and also a small test for it 🛠️.