Printing sources should display succinct column info

[hadley]

From Dave Cooper:

h = function(x, r=6, c=8) {
  if (is.null(dim(x))) {
    cat(format(x[1:min(r, length(x))], digits=2), '...')
    cat('\nlength=', length(x), ', class=', class(x), '\n', sep='')
  }
  else {
    print(x[1:min(r, nrow(x)), 1:min(c, ncol(x))], digits=2)
    cat('dim = (', dim(x)[1], ', ', dim(x)[2], '), class=', class(x), '\n', sep='')