Make C level `vec_proxy_equal()` recursive by default

[DavisVaughan]

See #636 (review)

It is not currently used non-recursively anywhere anymore, so we might not even need vec_proxy_equal_shallow().

[lionel-]

It seems like the R level vec_proxy() should also be recursive. And then restoration as well.

[lionel-]

E.g. there might be a grouped-df column whose group indices need to be restored after slicing.

[lionel-]

oops vec_proxy() cannot be recursive because vec_restore() cannot be recursive. We can't restore recursively since the number of columns or their contents might change.

It's possible vec-proxy and vec-proxy-equal are in fact unrelated concepts.

[lionel-]

vec_proxy_compare() also needs some clean up

test <- new_data_frame(n = 2L)
test$x <- new_data_frame(list(y = matrix(1:2, 2)))
vec_proxy_compare(test)
#>   V1
#> 1  1
#> 2  2

test$x <- tibble::tibble(y = matrix(1:2, 2))
vec_proxy_compare(test)
#> Error: No common type for `..1$y` <data.frame<V1:integer>> and `..2$y` <integer[,1]>.