feat(rust,python,cli): add LEFT string function for SQL

[alexander-beedie]

Adds SQL support for the string left function, and associated unit tests.

Example

import polars as pl
df = pl.DataFrame({
    "scol": ["abcde", "abc", "a", None],
})
with pl.SQLContext( frame=df ) as ctx:
    res = ctx.execute(
        'SELECT scol, LEFT(scol,2) AS "scol:left2" FROM frame',
    ).collect()
    
# shape: (4, 2)
# ┌───────┬────────────┐
# │ scol  ┆ scol:left2 │
# │ ---   ┆ ---        │
# │ str   ┆ str        │
# ╞═══════╪════════════╡
# │ abcde ┆ ab         │
# │ abc   ┆ ab         │
# │ a     ┆ a          │
# │ null  ┆ null       │
# └───────┴────────────┘