I'm probably going too far at this point

...but this is part of a well-intentioned effort to clear out some of
the "noise" of the repo. I'm thinking most of this stuff can probably go
into the gh-pages branch, or anyway, a *different* branch.
added pluck-style behavior for map (half-finished)
there, that's more professional!
well, it was fun while it lasted

...but I think using Sequence.define internally was a mistake. Maybe
I'll explain why in a blog post or something at some point? Anyway, live
and learn.
added toComprise matcher for testing sequences against arrays
added pluck-style callback for filter
added where-style callback for filter

also, added createCallback method