Conservatively export all trait methods and impls

The comments have more information as to why this is done, but the basic idea is
that finding an exported trait is actually a fairly difficult problem. The true
answer lies in whether a trait is ever referenced from another exported method,
and right now this kind of analysis doesn't exist, so the conservative answer of
"yes" is always returned to answer whether a trait is exported.

Closes #11224
Closes #11225
Don't leave lingering files in doc tests

Closes #11234
auto merge of #11245 : alexcrichton/rust/issue-11225, r=pcwalton

Commit messages are a little more descriptive.