modify extractors functions to work with altrep_sparse vectors

[EmilHvitfeldt]

To close #8

library(sparsevctrs)


vec <- new_sparse_real(c(1, 6, 30, 2), c(2, 4, 6, 9), 10)

vec
#>  [1]  0  1  0  6  0 30  0  0  2  0

sparsevctrs:::.values(vec)
#> [1]  1  6 30  2
sparsevctrs:::.positions(vec)
#> [1] 2 4 6 9
length(vec)
#> [1] 10

^{Created on 2024-04-30 with reprex v2.1.0}