RFC: Move `tri`, `four`, and `rope` out of libstd

[marijnh]

These don't seem to be generally useful enough to merit a place in the stdlib (and thus putting the maintenance burden on the core team). Does anyone object to removing them?

[pcwalton]

Ropes are nice for buffered I/O, assuming they do what I assume they do. Most people doing I/O with libuv will want them, actually. They're pretty much designed for reading bytes off the network in an asynchronous fashion.

[graydon]

Yeah, keep rope. The others seem pretty rare.

[catamorphism]

Interestingly, tri duplicates part of the tritv module that typestate uses, and it might be more efficient.