bpo-46845: Reduce dict size when all keys are Unicode.

[methane]

https://bugs.python.org/issue46845

[methane]

pyperformance (with --enable-optimizations --with-lto):

Result

Benchmark	main-opt	compact-opt
2to3	294 ms	292 ms: 1.01x faster
chameleon	7.78 ms	7.66 ms: 1.02x faster
chaos	81.0 ms	82.3 ms: 1.02x slower
crypto_pyaes	93.1 ms	94.3 ms: 1.01x slower
deltablue	4.59 ms	4.73 ms: 1.03x slower
dulwich_log	86.6 ms	85.3 ms: 1.02x faster
fannkuch	433 ms	426 ms: 1.02x faster
float	88.0 ms	86.5 ms: 1.02x faster
go	160 ms	158 ms: 1.01x faster
hexiom	7.40 ms	7.45 ms: 1.01x slower
json_dumps	13.8 ms	13.6 ms: 1.01x faster
json_loads	27.3 us	26.9 us: 1.02x faster
logging_format	7.38 us	7.33 us: 1.01x faster
logging_silent	118 ns	119 ns: 1.00x slower
logging_simple	6.72 us	6.66 us: 1.01x faster
mako	11.8 ms	11.5 ms: 1.02x faster
meteor_contest	116 ms	115 ms: 1.01x faster
nbody	101 ms	103 ms: 1.02x slower
pickle	13.1 us	13.0 us: 1.00x faster
pickle_dict	36.4 us	37.9 us: 1.04x slower
pickle_list	5.28 us	5.22 us: 1.01x faster
pidigits	216 ms	217 ms: 1.01x slower
pyflate	488 ms	490 ms: 1.00x slower
python_startup	28.5 ms	28.1 ms: 1.01x faster
python_startup_no_site	6.89 ms	6.80 ms: 1.01x faster
raytrace	347 ms	355 ms: 1.02x slower
regex_compile	157 ms	157 ms: 1.00x faster
regex_dna	233 ms	234 ms: 1.01x slower
regex_effbot	3.51 ms	3.46 ms: 1.01x faster
regex_v8	26.0 ms	25.5 ms: 1.02x faster
richards	54.8 ms	54.2 ms: 1.01x faster
scimark_fft	365 ms	368 ms: 1.01x slower
scimark_lu	130 ms	126 ms: 1.03x faster
scimark_monte_carlo	79.2 ms	77.7 ms: 1.02x faster
scimark_sor	137 ms	136 ms: 1.01x faster
scimark_sparse_mat_mult	5.44 ms	5.37 ms: 1.01x faster
spectral_norm	115 ms	116 ms: 1.01x slower
sqlalchemy_declarative	159 ms	158 ms: 1.01x faster
sqlalchemy_imperative	24.4 ms	24.2 ms: 1.01x faster
sqlite_synth	2.81 us	2.75 us: 1.02x faster
sympy_expand	566 ms	563 ms: 1.01x faster
sympy_integrate	23.9 ms	24.0 ms: 1.01x slower
sympy_sum	197 ms	196 ms: 1.00x faster
sympy_str	347 ms	344 ms: 1.01x faster
telco	7.50 ms	7.36 ms: 1.02x faster
unpack_sequence	52.3 ns	51.9 ns: 1.01x faster
unpickle	16.0 us	15.1 us: 1.06x faster
unpickle_list	5.42 us	5.28 us: 1.03x faster
unpickle_pure_python	274 us	271 us: 1.01x faster
xml_etree_generate	86.9 ms	86.3 ms: 1.01x faster
xml_etree_process	61.8 ms	61.6 ms: 1.00x faster
Geometric mean	(ref)	1.01x faster

Benchmark hidden because not significant (8): django_template, html5lib, nqueens, pathlib, pickle_pure_python, tornado_http, xml_etree_parse, xml_etree_iterparse

[methane]

sys.getsizeof(dict.fromkeys("abcdefg")) becomes 272 bytes from 352 bytes on 64bit platform.

maxrss(KB) of /usr/bin/time python -c 'import flask,sqlalchemy,fastapi'

	main	compact	-	ratio
default	46716	45300	1416	96.97%
-OO	43476	40844	2632	93.95%
-OO + nodebugrange	41424	39632	1792	95.67%

[methane]

This PR includes fixing dict_merge, dict_keys, dict_values, and dict_items don't respect order of split table.
They just use _PyDict_Next() for now.

[markshannon]

Thanks for fixing the regressions in split table ordering.