This allows simple enums, like Option<u32>, to work, since they already have the variant fields in the enum itself
(thanks to ScalarPair ABI - but this could easily be extended to at least all Option<T> without much effort).

for loops that rely on those Option types also work, except Range's implementation of Iterator uses mem::swap, and that happens to have a pretty severe case of microoptimization, that we'll have to turn off upstream (frankly, given that this kind of library optimization blocks MIR optimizations, generating that SIMD loop should probably be done in the backend, not in a library).

You can see a working example of a for loop with a custom Range-like iterator, in the third commit.