chore(rust): Replace num with num-traits + a few minor maintenance fixes

[aldanor]

• Drop num dependency in favor of num-traits (that's all that's being currently used anyway)
  • This removes four crates from the dependency tree: num, num-iter, num-bigint, num-rational
• Fix various build warnings appearing when some features are enabled/disabled
• Fix memchr version in polars-ops manifest
• Remove unneeded jemallocator dummy feature in polars-sql (also causing a build warning)
• In py-polars conversion, use pre-cached modules instead of importing them by hand on every conversion
• In py-polars conversion, query the type name of an object at most once if needed
• Update py-polars to 0.16.9 in Cargo.lock (not sure it should be bumped right away? but it comes up in git diff every time you run tests)

[ritchie46]

Thanks a lot. Sounds like a great maintenance run.