feat(rust,python): add dedicated name namespace for operations that affect expression names

[alexander-beedie]

Closes #11639.

As per discussion on the linked issue, this PR moves the following name-related expressions into a dedicated/new name namespace in Python and creates an equivalent namespace in Rust where these related expressions (and any future ones) can live:

Moved

• keep_name() → name.keep()
• map_alias() → name.map()
• prefix() → name.prefix()
• suffix() → name.suffix()

New

Now that the dedicated namespace exists (so there will be no confusion with the equivalent str.to_* methods), have also added the following new expressions:

• name.to_lowercase()
• name.to_uppercase()

Deprecation

• 🐍 Python: everything is gracefully deprecated as usual (will generate a warning).
• 🦀 Rust: will break existing usage of keep_name, suffix, prefix, map_alias; existing code will need to be updated to point at the relocated expressions in the new name expressions.

Misc

• Added new name namespace docs and tests, updated examples & user guide, etc.
• Minor tweak to avoid having both "name" & "names" modules in polars-plan/dsl; moved the one-line entry for COUNT into "consts" instead.

[ritchie46]

Nice! We need to wait merging this until the rust release is patched. But good to make it more meta.

[alexander-beedie]

Nice! We need to wait merging this until the rust release is patched. But good to make it more meta.

No problem; I'll keep it actively rebased (since it hits a lot of files!) until it's ready to roll ;)

[ritchie46]

Actually, the rust build is broken. We can merge this. :)