Unregister methods on unload

[hadley]

methods_register() also needs add a hook on package unload to remove all methods, and reset any unused hooks.

[mmaechler]

Relatedly, how can we removeMethod(), removeClass() for S7?

method(<gen>, <signature>) <-  NULL

would be R-like and not need a new function name, just an extension of existing method<-

[mmaechler]

... this is not urgent; there's also no API inverse of registerS3method().

[hadley]

Tracking the ability to unregister an individual method at #613

[hadley]

Also need to remove any hooks we added

[hadley]

Related: r-lib/pkgload#334