Initial submission.
Clarify that negative impls do not require the use of unsafe keyword.
Remove first-person language; cleanup typo
Incorporate description of `Copy` and `Sized` into the RFC so that it stands alone
Describe how to phase this in for 1.0
Merge remote-tracking branch 'nikomatsakis/opt-in-v2'
Opt-in-built-in traits v2 is RFC #3