Rework encode/decode functions to be more storage-friendly

[jphickey]

The BPv7 encode/decode functions currently operate on memory buffers. In order to facilitate other forms of storage, such as files, this needs to be further abstracted. The encode and decode functions should be reworked a bit to achieve:

• Support directly encoding/decoding from a file on disk as opposed to a memory buffer
• Not assume or require that the entire bundle or block is stored in a contiguous RAM block

This will be required to support file storage in an efficient manner.

[jphickey]

Adding this should permit the bundles to be stored in final/encoded form (CBOR) rather than the intermediate form they are stored in right now, which is larger than it needs to be.