Optimize strided loop assignment for runtime-contiguous layout types with contiguous assignment

[Ivorforce]

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Description

This is important for types that are dynamic statically, but still contiguous at runtime. I haven't tested runtime gains yet, but I'm guessing it should have an impact (especially by taking #2809 as a model).

The duplicated !E1::contiguous_layout logic is to give compilers an easier time to evaluate the if as a constexpr, deleting the uncontiguous path for clearly statically contiguous layouts.