Rename all files with the 'rc' extension
Add the `warnings` lint attribute
Deny common lints by default for lib{std,extra}
auto merge of #7113 : alexcrichton/rust/banned-warnings, r=cmr

Reopening of #7031, Closes #6963

I imagine though that this will bounce in bors once or twice... Because attributes can't be cfg(stage0)'d off, there's temporarily a lot of new stage0/stage1+ code.