implement abstractions Terminal and Buffer

Terminal encapsulates most of the already-existing logic for drawing things

Buffer corresponds directly to the user-facing concept of a buffer. it has facilities for creating a new empty one; for loading itself from a file; and for scanning its contents to compute the offsets to the start of each line.

at the moment, the assumption is that buffers are always entirely in-memory, but this will not always be true, so significant attention has been paid to aysnc behavior. there's locks in a few places which may or may not turn out to be how it ultimately works, but they seem like a credible first attempt.

the draw() routine is the heart of what exists so-far, doing all the really interesting stuff.

Force-Push: yes
Change-Id: Ifddc5debb12628233113c0bd6db3ea8cf10e6a5a

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