Use `vec_interleave()` in `pivot_longer()`

[DavisVaughan]

Right here:

tidyr/R/pivot-long.R

Lines 259 to 265 in d46d22a

	out <- vec_c(!!!val_cols, .ptype = val_type)
	# Interleave into correct order
	# TODO somehow `t(matrix(x))` is _faster_ than `matrix(x, byrow = TRUE)`
	# if this gets fixed in R this should use `byrow = TRUE` again
	n_vals <- nrow(data) * length(val_cols)
	idx <- t(matrix(seq_len(n_vals), ncol = length(val_cols)))
	vals[[value]] <- vec_slice(out, as.integer(idx))

As a follow up to #1132, which should make pivot_longer() a little faster, maybe do a benchmark against that PR.

Will require vctrs 0.3.9