Add with_mem_writer to std::rt::io::mem.

[brandonson]

This is in many ways a replacement for the current std::io::with_str_writer.

[brandonson]

Something similar is likely to be useful for moves to rt::io in many cases, and certainly in moving extra::json as discussed in #8679. I believe there are no plans for directly recreating the character and string i/o methods that are currently in std::io, but if this method or a similar one exists it would be possible to update code using with_str_writer by simply adding string-to-byte conversions.