Add a buffered stream interface

[alaviss]

Currently files only provide low-level read/write primitives. These are not buffered, so convenience like readLine are not available.

Goals:

• Pluggable buffering interface, basically so that any Stream can be plugged into BufferedStream and receive all the perks.

[c-blake]

You probably want to at least look at the POSIX getline/getdelim API. The reason is so that the control flow for the hot path can be "read buffer, memchr(..delim), maybe read more, ...". Doing this correctly can get you within around 1.25 .. 1.5x of memory mapped IO (which uses just the memchr part).

EDIT: you likely already know, but while this is a good thing to have, one of the downsides of this is that any FFI called/client code will not be able to "just use" the same "stdin/out/err" File objects, but will have to use your mirror of them for buffering to be coherent.