auto merge of #10466 : alexcrichton/rust/issue-10334, r=cmr

These commits create a `Buffer` trait in the `io` module which represents an I/O reader which is internally buffered. This abstraction is used to reasonably implement `read_line` and `read_until` along with at least an ok implementation of `read_char` (although I certainly haven't benchmarked `read_char`).

diff --cc src/libstd/io/mem.rs
Simple merge

diff --cc src/libstd/io/mod.rs
Simple merge