Change `GetManyMutError` to match T-libs-api decision

That is, differentiate between out-of-bounds and overlapping indices, and remove the generic parameter `N`.

I also exported `GetManyMutError` from `alloc` (and `std`), which was apparently forgotten.

Changing the error to carry additional details means LLVM no longer generates separate short-circuiting branches for the checks, instead it generates one branch at the end. I therefore changed the  code to use early returns to make LLVM generate jumps. Benchmark results between the approaches are somewhat mixed, but I chose this approach because it is significantly faster with ranges and also faster with `unwrap()`.

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