`read`, `write` return value when `n == buffer_size(buffers)`

[pdimov]

High level composition algorithms such as when_any and when_all need to be able to distinguish, in a generic manner, between success and failure of their subtasks. However, the case in which read or write returns [ec, n] when ec denotes failure, but n is equal to the requested amount of bytes to transfer, appears like an error to the generic observer, but is actually a success (and needs to be treated as one by high level compositions.)

For this reason, read and write need to be constrained to always return a successful ec when n equals buffer_size(buffers).

This also has implications for read_some and write_some, in terms of which read and write are implemented.