Add intrinsic for _slice_step

[tekknolagi]

This is part of the slicing machinery for all builtins and should be a
little boost.

[github-actions]

{
  "django_minimal_requests": {
    "benchmark": "django_minimal_requests",
    "cg_instructions before": 704063,
    "cg_instructions now": 704493,
    "cg_instructions ∆": "0.1%",
    "interpreter_args": [],
    "interpreter_name": "pyro",
    "version before": "29dde93d322fa79f734796cb5a356f348a3e3c77",
    "version now": "dbfd27c53e307e6c2bf8a9fd189c8114a7c5a0e8"
  }
}

[github-actions]

Summary

Metric	Average	Best	Worst	Notes
cg_instructions	-0.0%	fannkuch -0.4%	loadproperty 0.0%	typically < 0.2% noise

Benchmark details

Base vs. New

benchmark	cg_instructions
2to3	0.0%
bench_base64	0.0%
bench_compile	-0.0%
bench_pickle	-0.0%
deltablue	-0.0%
fannkuch	-0.4%
go	-0.0%
loadproperty	0.0%
nbody	0.0%
nqueens	0.0%
pyflate	0.0%
pystone	0.0%
richards	0.0%

CPython vs New

benchmark	cg_instructions
2to3	-10.1%
bench_base64	-38.7%
bench_compile	1106.8%
bench_pickle	-27.8%
deltablue	-64.5%
fannkuch	-1.8%
go	-64.1%
loadproperty	-75.2%
nbody	20.7%
nqueens	33.3%
pyflate	-30.6%
pystone	-71.8%
richards	-80.0%

Base

benchmark	cg_instructions
2to3	2,305,886,442
bench_base64	2,952,860,594
bench_compile	2,983,869,924
bench_pickle	2,990,796,668
deltablue	1,457,916,821
fannkuch	5,520,232,949
go	1,892,050,157
loadproperty	433,268,828
nbody	9,404,948,320
nqueens	2,950,778,123
pyflate	10,037,251,889
pystone	1,121,172,240
richards	963,344,004

New

benchmark	cg_instructions
2to3	2,305,896,779
bench_base64	2,952,888,226
bench_compile	2,983,711,955
bench_pickle	2,990,494,716
deltablue	1,457,896,779
fannkuch	5,496,009,799
go	1,892,027,502
loadproperty	433,278,464
nbody	9,404,955,772
nqueens	2,950,784,660
pyflate	10,037,254,806
pystone	1,121,175,857
richards	963,345,155

CPython

benchmark	cg_instructions
2to3	2,563,594,839
bench_base64	4,820,239,547
bench_compile	247,234,626
bench_pickle	4,143,855,350
deltablue	4,109,340,249
fannkuch	5,594,144,746
go	5,264,681,093
loadproperty	1,747,769,993
nbody	7,793,859,364
nqueens	2,213,172,543
pyflate	14,468,019,737
pystone	3,981,579,020
richards	4,821,556,215