add a struct ConsList<T> { Nil, Cons(T, ~List<T>) } library to libstd

[bblum]

Currently we have a bad one written in old idioms, using @-pointers, in libextra. This is wrong three ways.

[bblum]

Requires care about destruction. See #8399.

[thestinger]

I don't think this would really be useful since you'd almost always be better off using a vector.

[flaper87]

Triage bump. FWIW, I agree with @thestinger here.

I'll leave it open to get more feedback from the community

[thestinger]

It was actually decided to remove this rather than replace it with an owned list recently: #13183