Rewrite BUILD_SLICE to LOAD_SLICE_CACHED when it's constant

[tekknolagi]

This saves on allocations. It's a little finicky, though, since it
relies on a series of three LOAD_CONST with no EXTENDED_ARG.

[tekknolagi]

TODO: add tests

[tekknolagi]

An alternative idea: do this in the compiler. Unfortunately this means adding new marshaling and unmarshaling for slices and that seems more invasive.

[github-actions]

Summary

Metric	Average	Best	Worst	Notes
cg_instructions	0.5%	fannkuch 0.0%	nqueens 3.0%	typically < 0.2% noise

Benchmark details

Base vs. New

benchmark	cg_instructions
2to3	0.1%
bench_base64	0.2%
bench_compile	0.6%
bench_pickle	0.2%
deltablue	0.3%
fannkuch	0.0%
go	0.3%
loadproperty	1.1%
nbody	0.1%
nqueens	3.0%
pyflate	0.0%
pystone	0.4%
richards	0.5%

CPython vs New

benchmark	cg_instructions
2to3	-8.2%
bench_base64	-38.5%
bench_compile	1125.7%
bench_pickle	-19.1%
deltablue	-64.3%
fannkuch	-1.7%
go	-63.8%
loadproperty	-74.8%
nbody	22.2%
nqueens	46.6%
pyflate	-31.3%
pystone	-71.6%
richards	-79.9%

Base

benchmark	cg_instructions
2to3	2,351,294,592
bench_base64	2,960,306,334
bench_compile	3,014,236,993
bench_pickle	3,348,431,666
deltablue	1,462,407,738
fannkuch	5,500,944,998
go	1,901,340,490
loadproperty	435,662,650
nbody	9,515,556,698
nqueens	3,149,386,863
pyflate	9,942,548,610
pystone	1,124,779,200
richards	966,157,436

New

benchmark	cg_instructions
2to3	2,353,187,673
bench_base64	2,965,421,664
bench_compile	3,031,810,010
bench_pickle	3,353,877,017
deltablue	1,467,459,656
fannkuch	5,501,326,423
go	1,906,526,645
loadproperty	440,629,055
nbody	9,520,537,575
nqueens	3,245,419,927
pyflate	9,944,682,455
pystone	1,129,772,318
richards	971,169,185

CPython

benchmark	cg_instructions
2to3	2,563,516,982
bench_base64	4,820,256,525
bench_compile	247,353,166
bench_pickle	4,145,151,943
deltablue	4,109,392,466
fannkuch	5,594,123,299
go	5,264,677,544
loadproperty	1,747,788,203
nbody	7,793,833,304
nqueens	2,213,199,125
pyflate	14,468,078,381
pystone	3,981,557,801
richards	4,821,532,606

[tekknolagi]

Better idea: python/cpython#94168