Move CRC calculation from mpstream into encoder

[jphickey]

The CRC computations are currently part of the "mpstream" module, and is done when writing the content to the memory blocks.
However, as noted in issues #108 and #109, this constitutes and necessitates an extra copy of the entire bundle in memory. Ideally, the encoding could be done "direct to storage" and bypass this extra copy.
To enable this, the CRC calculation needs to move out of the mpstream module (which may not be in use) and into a separate entity in the encoder itself.