seq_along and rep_along

Hi all,

A couple of ideas for improving seq_along:

* It would be really useful to have a second argument dim:

     seq_along(mtcars, 1)
     seq_along(mtcars, 2)
     # equivalent to
     seq_len(dim(mtcars)[1])
     seq_len(dim(mtcars)[2])

   I often find myself wanting to iterate over the rows or column of a
data frame, and there isn't a particularly nice idiom if you want to
avoid problems with zeros - you have to use seq_len(nrow(df)) etc

* To me, it would seem be very natural to have a rep_along function:

   rep_along<- function(x, y) rep(x, length.out = length(y))

   possibly with more checking for the case where the lengths aren't
integer multiples.

I'd be happy to submit proposed implementations/documentation if there
was interest.

Hadley