Buffer-oriented synchronous read stream requirements

A type X meets the SyncReadStream 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 ec denotes an object of type error_code.

Table 28. SyncReadStream requirements

operation	type	semantics, pre/post-conditions
`a.read_some(mb)` `a.read_some(mb,ec)`	`size_t`	Meets the requirements for a read operation. If `buffer_size(mb) > 0`, reads one or more bytes of data from the stream `a` into the buffer sequence `mb`. If successful, sets `ec` such that `!ec` is `true`, and returns the number of bytes read. If an error occurred, sets `ec` such that `!!ec` is `true`, and returns 0. If all data has been read from the stream, and the stream performed an orderly shutdown, sets `ec` to `stream_errc::eof` and returns 0. If `buffer_size(mb) == 0`, the operation shall not block. Sets `ec` such that `!ec` is `true`, and returns 0.