gh-142889: Restructure PyDictKeysObject memory layout for simpler access

[clintonsteiner]

Restructure dict keys allocation to store dk_indices before the PyDictKeysObject header and keep dk_entries after the header.

Update dict index access and related allocation/free/clone paths, adjust gdb dict entry location logic, and add layout coverage tests.

Local dict microbenchmarks showed about a 1.4% overall improvement, with most operations around 1-2% faster.

• Issue: Improved performance and arguably simpler code for dictionaries by changing the keys layout #142889

[python-cla-bot]

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[bkap123]

Does this layout comment need updating (move dk_entries[] to the top and demark where a PyDictKeysObject * actually points to)?

Also, I feel like the definition for PyDictKeysObject (struct _dictkeysobject) needs updating:

cpython/Include/internal/pycore_dict.h

Lines 175 to 219 in 5944a53

	struct _dictkeysobject {
	Py_ssize_t dk_refcnt;
	/* Size of the hash table (dk_indices). It must be a power of 2. */
	uint8_t dk_log2_size;
	/* Size of the hash table (dk_indices) by bytes. */
	uint8_t dk_log2_index_bytes;
	/* Kind of keys */
	uint8_t dk_kind;
	#ifdef Py_GIL_DISABLED
	/* Lock used to protect shared keys */
	PyMutex dk_mutex;
	#endif
	/* Version number -- Reset to 0 by any modification to keys */
	uint32_t dk_version;
	/* Number of usable entries in dk_entries. */
	Py_ssize_t dk_usable;
	/* Number of used entries in dk_entries. */
	Py_ssize_t dk_nentries;
	/* Actual hash table of dk_size entries. It holds indices in dk_entries,
	or DKIX_EMPTY(-1) or DKIX_DUMMY(-2).
	Indices must be: 0 <= indice < USABLE_FRACTION(dk_size).
	The size in bytes of an indice depends on dk_size:
	- 1 byte if dk_size <= 0xff (char*)
	- 2 bytes if dk_size <= 0xffff (int16_t*)
	- 4 bytes if dk_size <= 0xffffffff (int32_t*)
	- 8 bytes otherwise (int64_t*)
	Dynamically sized, SIZEOF_VOID_P is minimum. */
	char dk_indices[]; /* char is required to avoid strict aliasing. */
	/* "PyDictKeyEntry or PyDictUnicodeEntry dk_entries[USABLE_FRACTION(DK_SIZE(dk))];" array follows:
	see the DK_ENTRIES() / DK_UNICODE_ENTRIES() functions below */
	};

Probably just change char dk_indices[] to char dk_entries[] and add a comment saying that dk_indicies will be put above the header. I'm not sure about the consequences of renaming dk_indices though, so you could also just add a comment.

[clintonsteiner]

Updated, was trying to keep diff small but agreed this is no longer applicable