Buffer-oriented asynchronous write stream requirements

A type X meets the AsyncWriteStream requirements if it satisfies the requirements listed below.

In the table below, a denotes a value of type X, cb denotes a (possibly const) value satisfying the ConstBufferSequence requirements, and t is a completion token.

Table 7. AsyncWriteStream requirements

operation	type	semantics, pre/post-conditions
`a.get_executor()`	A type satisfying the Executor requirements.	Returns the associated I/O executor.
`a.async_write_some(cb,t)`	The return type is determined according to the requirements for an asynchronous operation.	Meets the requirements for a write operation and an asynchronous operation with completion signature `void(error_code ec, size_t n)`. If `buffer_size(cb) > 0`, initiates an asynchronous operation to write one or more bytes of data to the stream `a` from the buffer sequence `cb`. If successful, `ec` is set such that `!ec` is `true`, and `n` is the number of bytes written. If an error occurred, `ec` is set such that `!!ec` is `true`, and `n` is 0. If `buffer_size(cb) == 0`, the operation completes immediately. `ec` is set such that `!ec` is `true`, and `n` is 0.