Rename operator+= for const_buffer and mutable_buffer #430

For a type that represents a sequence of byes bound from two sides, operator+= does not generalize from raw pointers unambiguously and has counterintuitive semantics:

1. operator+= associated with "increasing" actually shrinks the buffer.
2. std::string which can be made a buffer, uses operator+= for something completely different.

Please use a named member function for this purpose, like .consume_prefix().