Implement expression functions

[markfairbanks]

• [x] abs
• [x] between | pl.is_between
• [x] cast
• [x] count
• [x] cor
• [x] cov
• [ ] cum: max, mean, median, min, sum, count
• [x] floor
• [x] first/last
• [x] lag/lead
• [x] length
• [x] log/log10
• [x] is_finite/is_infinite
• [x] is_in/is_not_in
• [x] is_nan
• [x] is_not
• [x] is_null/is_not_null
• [x] max
• [x] mean
• [x] median
• [x] min
• [x] n_distinct | pl.n_unique
• [x] rep
• [ ] rolling_: apply, max, mean, median, min, sd
• [x] round
• [ ] sample
• [x] sd | pl.std
• [x] sqrt
• [x] sum
• [x] quantile
• [x] var