RFC: remove blanket ext implementations

[Rendered](https://github.com/skade/rfcs/blob/remove-blanket-ext-implementations/text/0000-remove-blanket-ext-implementations.md)

Previous discussions:
https://github.com/rust-lang/rust/issues/19435#issuecomment-65443991
https://github.com/rust-lang/rfcs/pull/445#issuecomment-65433687
https://github.com/rust-lang/meeting-minutes/blob/master/weekly-meetings/2014-11-25.md#extension-traits

Considering that I cannot get a good answer on how to work around issues with the new system, I think this is worth revisiting.

http://stackoverflow.com/questions/27267767/working-around-the-limitations-of-extension-traits