Consider adding noexcept to buffer sequence requirements

[chriskohlhoff]

[buffer.reqmts.mutablebuffersequence], [buffer.reqmts.constbuffersequence], [buffer.seq.access]

We should consider:

• Adding "Shall not exit via an exception." to the the requirements for net::buffer_sequence_begin(x) and 'net::buffer_sequence_end(x).
• Requiring that the conversion of the iterator value type to const_buffer or mutable_buffer should not exit via exception.
• (And perhaps place the same requirement on the iterator traversal and dereference too, although I'm not sure if this is already implied elsewhere in the standard?)
• Adding noexcept to the buffer sequence access functions.

The current implementation in asio assumes that these never throw, and in general I think low level buffer operations should not throw.