Improve decoder performance by 16-22%. See azul3d/audio#5

Before
BenchmarkDecodeFloat32	      30	 518081917 ns/op
BenchmarkDecodeFloat64	      30	 547400233 ns/op
BenchmarkDecodeUint8	      30	 491020468 ns/op
BenchmarkDecodeInt16	      30	 504408065 ns/op
BenchmarkDecodeInt24	      20	 657118984 ns/op
BenchmarkDecodeInt32	      30	 516224602 ns/op
BenchmarkDecodeALaw	      30	 533577493 ns/op
BenchmarkDecodeMuLaw	      30	 488511090 ns/op

After
BenchmarkDecodeFloat32	      30	 414876430 ns/op
BenchmarkDecodeFloat64	      30	 431367131 ns/op
BenchmarkDecodeUint8	      30	 390492699 ns/op
BenchmarkDecodeInt16	      30	 409407364 ns/op
BenchmarkDecodeInt24	      30	 539338196 ns/op
BenchmarkDecodeInt32	      30	 416405221 ns/op
BenchmarkDecodeALaw	      30	 445839555 ns/op
BenchmarkDecodeMuLaw	      30	 392282710 ns/op

The performance gain comes from removing binary.Size calls in the decoder's tight loops.