feat(python,rust,cli): add SQL support for null-aware equality checks

[alexander-beedie]

Now that we have expressions that distinguish between null-aware/unaware comparison, we can integrate the same into our SQL support; this PR covers both the spaceship operator <=>, and is [not] distinct from expressions.

Example

import polars as pl

df = pl.DataFrame({
    "a": [1, None, 3, 6, 5], 
    "b": [1, None, 3, 4, None],
})

with pl.SQLContext( frame_data=df ) as ctx:
    out = ctx.execute(
        """
        SELECT
          -- not null-aware
          (a = b)  as "1_eq_unaware",
          (a != b) as "2_neq_unaware",
          -- null-aware
          (a <=> b) as "3_eq_aware",
          (a IS NOT DISTINCT FROM b) as "4_eq_aware",
          (a IS DISTINCT FROM b) as "5_neq_aware",
        FROM frame_data
        """
    ).collect()

# shape: (5, 5)
# ┌──────────────┬───────────────┬────────────┬────────────┬─────────────┐
# │ 1_eq_unaware ┆ 2_neq_unaware ┆ 3_eq_aware ┆ 4_eq_aware ┆ 5_neq_aware │
# │ ---          ┆ ---           ┆ ---        ┆ ---        ┆ ---         │
# │ bool         ┆ bool          ┆ bool       ┆ bool       ┆ bool        │
# ╞══════════════╪═══════════════╪════════════╪════════════╪═════════════╡
# │ true         ┆ false         ┆ true       ┆ true       ┆ false       │
# │ null         ┆ null          ┆ true       ┆ true       ┆ false       │
# │ true         ┆ false         ┆ true       ┆ true       ┆ false       │
# │ false        ┆ true          ┆ false      ┆ false      ┆ true        │
# │ null         ┆ null          ┆ false      ┆ false      ┆ true        │
# └──────────────┴───────────────┴────────────┴────────────┴─────────────┘

[ritchie46]

Nice.. learn more of SQL every day.

[alexander-beedie]

The "spaceship" operator has such a great name...
(You can see how I stumbled across the eq_missing doc entry now too ;)

[ritchie46]

The "spaceship" operator has such a great

Reminds me of rusts "turbofish" ^^