Specialized `match_keys` for exact dictionary type

Feature or enhancement

I think it's probably worthwhile to write a specialized version of match_keys (for structural pattern matching of mappings) for exact dictionary types

Pitch

The most common mapping type is almost certainly the built-in dict. The match_keys function in ceval.c could be specialized to handle this type. Doing so would allow you to skip the dummy object, and replace the get call with PyDict_GetItemWithError. I don't think this should change the observable behaviour at all for exact dict

I'm in the process of reimplementing structural pattern matching on Cython, and this change gives ~2x optimization of some microbenchmarks.

If this change is deemed desirable I'm happy to implement it myself.

Previous discussion

Not aware of any

Inizia in ceval.c, all'interno di match_keys, e verifica come lo structural pattern matching gestisce attualmente le chiavi delle mapping e i tipi dict esatti. La modifica è completa quando i dict esatti usano il percorso specializzato senza modificare il comportamento osservabile, inclusa la semantica di lookup esistente; confronta i microbenchmark pertinenti, se disponibili.