Re-introduce the "expensive debug assertions" feature

[RalfJung]

IIRC we used to have a cargo feature to control whether we run the Stacked Borrows cache consistency checks. At some point I replaced that by cfg!(debug_assertions). For debug builds of Miri, the assertions don't make a huge difference. (In a quick benchmark with time ./miri test, I can't even see a difference.)

But we're getting Miri to work better in x.py, and that tends to be configured to use release builds with debug assertions. I don't think we want the cache consistency check running as part of ./x.py test library/core --miri, even if one has debug assertions enabled for the compiler. So I think we should go back to using an off-by-default feature flag for this.