`parser::read_some` should take a generic mutable buffer sequence instead of a span

[ashtum]

parser::read_some currently takes std::span<capy::mutable_buffer const>, so callers must first materialize an array of descriptors. Instead, it should be a constrained template over capy::MutableBufferSequence that iterates over the sequence and forwards each non-empty buffer to a single-buffer, out-of-line implementation.