decoder: Avoid binary.Read / improve performance by 55-88%.

benchmark                  old ns/op     new ns/op     delta
BenchmarkDecodeFloat32     14861757      5489679       -63.06%
BenchmarkDecodeFloat64     16008682      7067748       -55.85%
BenchmarkDecodeUint8       13951858      4908209       -64.82%
BenchmarkDecodeInt16       14685837      5394720       -63.27%
BenchmarkDecodeInt24       50474652      5591962       -88.92%
BenchmarkDecodeInt32       15584887      5472800       -64.88%
BenchmarkDecodeALaw        14637388      4956828       -66.14%
BenchmarkDecodeMuLaw       14626045      4966246       -66.05%

Helps #8