feat: Don't allocate another DType in `DType::eq_ignore_nullability` #6642

Job	Run time
Python (test)	6m 35s
Python (lint)	3m 48s
Rust build (all-features)	9m 42s
bench test	19m 8s
Check generated proto/fbs files are up to date	48s
Rust build (default features)	7m 33s
License Check and Audit Check	5m 54s
Rust build (with tokio dispatcher)	9m 4s
Rust build (with compio dispatcher)	8m 21s
miri	8m 9s
Rust (docs)	2m 28s
Rust (lint)	7m 7s
Rust build (with tokio+compio dispatcher)	7m 58s
Rust build (wasm32 with default features)	3m 47s
Rust (tests)	11m 10s
	1h 51m 32s

Provide feedback