Nonstandard _sub(), _setSub() and _concat() for RawData

[marcgurevitx]

Adding new nonstandard methods for RawData:

• _sub(): Returns a fragment of RawData as another RawData object.
• _setSub(): Rewrites a frament of RawData with another RawData.
• _concat(): Joins several RawData objects into a single one (a class method).

I don't insist on merging. Just posting for review (and better method names suggestions maybe.)