Reduce number of copy operations in Decode

[karaketir16]

DecodeSingle gets a single slice, data combined with ecc. If possible, using this function instead of Decode reduces the memory usage and allocation.

goos: linux
goarch: amd64
pkg: github.com/maruel/rs
cpu: 12th Gen Intel(R) Core(TM) i5-12450H
BenchmarkDecode16_10-12                     	 2507281	       479.0 ns/op	83751332.47 MB/s	      48 B/op	       2 allocs/op
BenchmarkDecodeSingle16_10-12               	 2553074	       457.3 ns/op	89333181.87 MB/s	      16 B/op	       1 allocs/op
BenchmarkDecode16_10With1Error-12           	 1266762	      1037 ns/op	19535895.82 MB/s	     512 B/op	      27 allocs/op
BenchmarkDecodeSingle16_10With1Error-12     	 1000000	      1006 ns/op	15899815.31 MB/s	     480 B/op	      26 allocs/op
BenchmarkDecode128_16-12                    	  200122	      6170 ns/op	4151483.66 MB/s	     160 B/op	       2 allocs/op
BenchmarkDecodeSingle128_16-12              	  202068	      6001 ns/op	4309959.18 MB/s	      16 B/op	       1 allocs/op
BenchmarkDecode128_16With1Error-12          	  178846	      6609 ns/op	3463615.24 MB/s	     662 B/op	      27 allocs/op
BenchmarkDecodeSingle128_16With1Error-12    	  180782	      6521 ns/op	3548558.03 MB/s	     518 B/op	      26 allocs/op
BenchmarkDecode128_16With2Error-12          	  163591	      7252 ns/op	2887260.40 MB/s	    1032 B/op	      45 allocs/op
BenchmarkDecodeSingle128_16With2Error-12    	  164965	      7267 ns/op	2905602.99 MB/s	     888 B/op	      44 allocs/op
PASS
ok  	github.com/maruel/rs	14.251s

[karaketir16]

As I said in #3 (comment), I can not modify the poly. I am keeping two functions for now.