Buffer-oriented asynchronous read stream requirements

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

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

Table 5. AsyncReadStream requirements

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