Added overview of the debuginfo module at the beginning of debuginfo.rs.
Fixed compiler warnings about unnecessary string allocations in debuginfo.rs
Reordered debuginfo.rs and commented public fns
debuginfo.rs: Removed some static constants not used anymore.
debuginfo.rs: Explained why a function is called although one could think it should not be.
debuginfo.rs: Removed trailing whitespace.
Make task_rng() return @mut R instead of @@mut R
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.
auto merge of #7216 : kballard/rust/task_rng, r=brson
auto merge of #7255 : michaelwoerister/rust/debuginfo, r=jdm

This PR contains no real code changes. Just some documentation additions in the form of comments and some internal reordering of functions within debuginfo.rs.
Turn on using LLVM threadsafely