This commit changes the iterator category to andom_access_iterator and allows offsets and subscript operators for object iterators.

The CBOR and MessagePack parsers now expect the input to be read until the end. Unless the new parameter "strict" is set to false (it is true by default), an exception is raised if the parser ends prematurely. This is a breaking change as the parsers ignored unread input so far.

Furthermore, the offset/startIndex paramter introduced in #462 was removed as this behavior can be mimicked with an iterator range. For instance, instead of calling "from_cbor(vec, 5);", you can write "from_cbor({vec.begin()+5, vec.end()});".

Added a parameter to control whether parsing CBOR and MessagePack must completely consume the input.