frame: Create dedicated file for frame header code.
frame: Create dedicated file for subframe code.
frame: Initial parsing of subframes implemented, read more below.
meta: Minor alteration of comment.
rsf: Start using frame.NewFrame.

The changes to frame are as follows:
	* Frame struct no longer contains Footer. Instead NewFrame returns an error
	  if the CRC-16 verification fails.
	* Implement NewFrame which parses audio frames. Each audio frame consists of
	  a frame header, one subframe per channel and a footer containing a CRC-16
	  checksum.
	* Initial implementation of NewSubFrame which parses subframes. Currently
	  only the easiest case is fully supported.
		- Constant encoding is fully supported.
		- Fixed encoding can currently decode the "warm-up" samples. Rice0 and
		  rice1 still have to be implemented for full support.
		- LPC - not yet implemented.
		- Verbatim - not yet implemented. This one should be easy though.
	* NewSubHeader has almost been fully implemented. Only
	  "wasted-bits-per-sample" remains.

Note: This code uses the bitstream Reader from "github.com/mewkiz/pkg/bit",
which is convenient but also inefficient. When decoding has been fully
implemented we should use profiling to find the bottlenecks.