Improve decoder performance by 9-11%. See azul3d/audio#5

Before
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

After
BenchmarkDecodeFloat32	      20	 377842782 ns/op
BenchmarkDecodeFloat64	      20	 392514910 ns/op
BenchmarkDecodeUint8	      20	 359224923 ns/op
BenchmarkDecodeInt16	      20	 374009324 ns/op
BenchmarkDecodeInt24	      20	 495933463 ns/op
BenchmarkDecodeInt32	      20	 372523136 ns/op
BenchmarkDecodeALaw	      20	 409146299 ns/op
BenchmarkDecodeMuLaw	      20	 360234622 ns/op

This is done by simply moving the variable declarations outside of
the tight decoding loops.