Allow manual implementations of built-in traits

[RFC #3]

cc #13231
Allow built-in traits to be derived

[RFC #3]

cc #13231
auto merge of #13868 : FlaPer87/rust/opt-in-phase1, r=alexcrichton

This is a first patch towards an opt-in built-in trait world. This patch removes the restriction on built-in traits and allows such traits to be derived.

[RFC#3]

cc #13231

@nikomatsakis r?