Rename `transpose()` to `list_transpose()`

[hadley]

And broadly reconsider its scope and implementation given vctrs and tidyr.

Related issues:

• Transpose a list with named elements where the names don't correspond #397 — doesn't work if names don't align
• Transpose drops factors #471 — only works for atomic vectors

[hadley]

One possible implementation would be to use map_vec() to get automatic simplification, something like this:

list_transpose <- function(x) {
  map(vec_index(x), ~ map_vec(x, .x))
}

Or maybe:

list_transpose <- function(x, template = vec_index(x), .ptype = NULL) {
  map(index, ~ map_vec(x, .x, .ptype = .ptype[.x]))
}

This will be much slower than the existing transpose() implementation, but it'll be vastly more robust.