Only use fast_disk_diag for big.matrices

* ff matrices have a method for vector indices that is faster
* big.matrix actually has a method for 
  x[ m ] where m is a 2 col matrix, but I can’t get it to work!
* see commit 3820fffbc52cffd21d9760dd4365c9423e3f283b
Consistently use rownames to name diagonal
Make diagonal a generic method and export it