[boost] Re: [Capy] Review Note

1. Buffer sequences

* ConstBufferSequence, MutableBufferSequence concepts * supporting primitives: buffer_begin, buffer_end, buffer_size, buffer_length, buffer_slice

Incidentally, `buffer_begin` is not named `buffer_begin` in Capy, but `begin`. This is a mistake. If `x` is `span<byte>` and we decide that spans are buffers, `begin(x)` is `x.begin()`, but `buffer_begin(x)` is `&x`. These two operations are not the same thing, so they should not be called with the same name.