Unexport unused s3 methods

[MichaelChirico]

Part of #76.

Unfortunately, I've learned that some primitives, e.g. -, don't appear on the call stack when checked from a corresponding method:

`-.foo` = function(x) sys.calls()
`-`(structure(1L, class='foo'))
# [[1]]
# `-.foo`(structure(1L, class = "foo"))

Therefore I'll need to simply un-export these ones directly. I'll do so in a separate PR.