Add experimental fallible::Vec

[ChrisDenton]

Tracking issue: #157392

There are a few things to be aware of here:

1. This is still experimental. It might be removed at any time so we have to be careful about any interdependency between this and Vec
2. Vec is a pretty crucial part of rust's standard library. We have to be extremely careful touching its code. We might regress performance or make debugging worse.

With that in mind, this PR does the minimum necessary for an MVP fallible::Vec. The only actually new thing it adds is fallible push and push_mut methods. The rest just forwards to Vec equivalents. Further APIs can be added (carefully) afterwards.

I apologise for the number of lines changed in the PR but they are mostly copy/pasted docs (I wanted them for the testable examples mostly). We should probably find a better way of duplicating them but I think this works for a minimally intrusive experiment.

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[ChrisDenton]

 -	   = note: multiple `impl`s satisfying `Vec<_>: aux::Trait` found in the `unstable_impl_method_selection_aux` crate:
 -	           - impl aux::Trait for Vec<u32>;
 -	           - impl aux::Trait for Vec<u64>
 +	   = note: multiple `impl`s satisfying `std::vec::Vec<_>: aux::Trait` found in the `unstable_impl_method_selection_aux` crate:
 +	           - impl aux::Trait for std::vec::Vec<u32>;
 +	           - impl aux::Trait for std::vec::Vec<u64>

Ooph, that's unfortunate. Simplest thing is to rename this type. But I do wonder if that could be fixed.