Resolves dotnet#78681 (or "there's nothing else we'd be willing to do for it"). This is the rest of what I implemented trying to get `SearchValues.Create(someVeryLongString)` to preinitialize. It doesn't actually enable more `SearchValues` scenarios because I eventually hit codepaths that would require us to implement hardware intrinsics support in the interpreter. The `SearchValues` scenarios that we do support were implemented in dotnet#92470 and dotnet#92666. But since I already implemented this, here it is; maybe it will be useful for something else:

* Support for modelling `Span<X>`, including creating it from stackalloc
* Support for math/comparisons with native int
* `Unsafe.Add`