Use implicitGeneric

[PeteHaitch]

If I understand this email from Martin Maechler, I should be using implicitGeneric() to create S4 generics for functions in matrixStats https://www.mail-archive.com/[email protected]/msg08422.html

I may need to re-think dispatch on the seed and it's pristine-ness. E.g., I may still need to export a matrix-method for each of these S4 generics to avoid the ANY-method. And I may need to switch use of hasMethod() to the stricter existsMethod().