allow ^D (EOF) to exit repl (instead of going into an infinite loop)
report error instead of crashing when magpie can't find "core" module in repl
Add type guard.
Merge branch 'master' of https://github.com/joshuawarner32/magpie
Clean up a bit.
Add skip() on Iterables.

Also get rid of generate(). It's a neat idea, but doesn't have the
right semantics since fibers are eager. Something like generate()
could be created, but it will need a pair of channels to ensure the
generator doesn't run ahead of the thing pulling from it.
Sketch in some float arithmetic tests.