Rename operator+= for const_buffer and mutable_buffer

[akrzemi1]

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().

[akrzemi1]

remove_prefix sounds like a natural candidate with a precedent.

[akrzemi1]

Name .advance() has been suggested as an alternative. I recommend against it. It implies that we are talking about a pointing device: something indicating position (like a pointer). But a buffer is not a position: it is a span. A span can be widened or shortened, but not advanced.

[akrzemi1]

The suggestion for advance() comes from Rust's Buf. However, the naming choice in Rust should be considered in the context of Rust and Rust's Buf, while the choice in Capy should be considered in the contect of C++ and Capy's const_buffer and ConstBufferSequence.

In Rust Buf itself is defined as "a cursor into the buffer". In my opinion this framing itself is incorrect (as explained above), but within this framing "advance" -- or even += -- makes sense.

Another note is that Rust's Buf is an equivalent of Capy's ConstBufferSequence. So the name advance wuld be an alternative to consuming_buffers::consume.