Use ALTREP safe operations

[mitchelloharawild]

For instance, viewing a tibble unnecessarily evaluates the sequences:

library(vecvec)
x <- tibble::tibble(x = vecvec(seq_len(1e6)), y = seq_len(1e6))
pryr::object_size(x$x)
#> 32.90 kB
pryr::object_size(x$y)
#> 680 B
x
#> # A tibble: 1,000,000 × 2
#>    x           y
#>    <int*>  <int>
#>  1       1     1
#>  2       2     2
#>  3       3     3
#>  4       4     4
#>  5       5     5
#>  6       6     6
#>  7       7     7
#>  8       8     8
#>  9       9     9
#> 10      10    10
#> # ℹ 999,990 more rows
pryr::object_size(x$x)
#> 4.03 MB
pryr::object_size(x$y)
#> 680 B

^{Created on 2026-04-10 with reprex v2.1.1}

[mitchelloharawild]

This seems to be happen in pillar::df_head -> ... -> pillar::vec_head -> vctrs::vec_slice.
vec_slice() can fallback to [, but if vec_proxy() exists then it won't - maybe vctrs methods (e.g. proxy) need to be dropped?

edit: the implementation of vec_proxy() for vecvec seems ALTREP safe, it must be later in slicing. vec_restore() is almost certainly not ALTREP safe -- confirmed, vec_group_loc on x values ends ALTREP. The main need for proxy+restore is vec_c() compatibility, perhaps there is a better proxy that is more easily restored?

[mitchelloharawild]

format() also seems to destroy ALTREP - that should be easier to fix.