DecodeUlawRead and DecodeAlawRead to accept buffer instead returning

[emiago]

Add
DecodeUlaw and DecodeAlaw as one more alternative where you accept buffer for PC instead returning it.

Caller can make reuse of buffer memory, considering this are uncompressed formats and size can be calculated.
Should be small fix to add?

[zaf]

It will be addressed here: #3
It introduces functions that use user defined buffers:

BenchmarkEncodeUlaw-4     	   13824	     86166 ns/op	 472.39 MB/s	   20480 B/op	       1 allocs/op
BenchmarkEncodeUlawTo-4   	   14311	     82727 ns/op	 492.03 MB/s	       0 B/op	       0 allocs/op
BenchmarkDecodeUlaw-4     	   35204	     33762 ns/op	 602.82 MB/s	   40960 B/op	       1 allocs/op
BenchmarkDecodeUlawTo-4   	   45838	     25614 ns/op	 794.58 MB/s	       0 B/op	       0 allocs/op
BenchmarkUlaw2Alaw-4      	   78462	     14701 ns/op	1384.36 MB/s	   20480 B/op	       1 allocs/op
BenchmarkUlaw2AlawTo-4    	  101139	     11891 ns/op	1711.47 MB/s	       0 B/op	       0 allocs/op