feat(rust,python,cli): add LENGTH and OCTET_LENGTH string functions for SQL

[alexander-beedie]

Support for SQL string length functions, in terms of both characters (length) and bytes (octet_length).

Also:

• adds the related functions into StringNameSpace on the Rust side.
• sorts some string function definitions for clarity).

Example

import polars as pl

df = pl.DataFrame({
    "words": ["Café", None, "東京"],
})

with pl.SQLContext( frame=df ) as ctx:
    res = ctx.execute(
        """
        SELECT
          words,
          LENGTH(words) AS n_chars,
          OCTET_LENGTH(words) AS n_bytes
        FROM frame
        """
    ).collect()

# shape: (3, 3)
# ┌───────┬─────────┬─────────┐
# │ words ┆ n_chars ┆ n_bytes │
# │ ---   ┆ ---     ┆ ---     │
# │ str   ┆ u32     ┆ u32     │
# ╞═══════╪═════════╪═════════╡
# │ Café  ┆ 4       ┆ 5       │
# │ null  ┆ null    ┆ null    │
# │ 東京  ┆ 2       ┆ 6       │
# └───────┴─────────┴─────────┘

[ritchie46]

Nice, can we now properly dispatch the python side to those?

[alexander-beedie]

Oops! Missed the final link in the chain 😅 Should be good now - jumping on a plane in about an hour (at the airport now), so if there's anything else then I'll pick it up when I land and get home this evening. (In the meantime, let's see if I can get anything done from the emergency exit row at 10km up :)

[ritchie46]

All good! You will land in a world with this PR merged. ;)