collections: remove List

It was decided in a meeting that this module wasn't needed,
and more thought should be put into a persistent collections
library.
test: remove pure test, which is now redundant with inline tests
std and green: fix some warnings
auto merge of #13183 : erickt/rust/remove-list, r=alexcrichton

`collections::list::List` was decided in a [team meeting](https://github.com/mozilla/rust/wiki/Meeting-weekly-2014-03-25) that it was unnecessary, so this PR removes it. Additionally, it removes an old and redundant purity test and fixes some warnings.