More lazy functions

[sorhawell]

• add missing lazy functions to pl$...
• rename functions_x files to match py-polars
• deprecate $list()
• add n_unique, approx_n_unique, head, tail

[sorhawell]

remainder not implement plus some priortization 3 higher

3#map
3#apply
3#fold
3#reduce
3#cumfold
3#cumreduce
3#exclude
3#groups
3#format

2#cumsum
2#any
2#all cover bitwise also
2#quantile
2#arange
2#pl$collect_all
2# pl$repeat
2#pl$arg_where

1#spearman_rank_corr
1#pearson_corr
1#corr
1#cov
1#arg_sort_by
1#duration
1#datetime_
1#date_
1#concat_str_
1#pl$from_epoch
1#pl$rolling_cov
1#pl$rolling_corr

[vincentarelbundock]

If NULL is not an acceptable input, then we may want to leave the argument without default and include an is.missing() check instead.

[sorhawell]

oups good catch :)

[vincentarelbundock]

remainder not implement plus some priortization 3 higher

I'm likely have time to look at some of these next week if you are willing to wait. I can open a separate PR.

[sorhawell]

I'm likely have time to look at some of these next week if you are willing to wait. I can open a separate PR.

Cool @vincentarelbundock :), I made a task list in #204. You can reserve any you want, but it would anyways be unlikely I would get to finish them all by next week.
I will also wrap up this PR and start a new one.