Removed drop from the keyword list in the docs
auto merge of #7582 : mitsuhiko/rust/doc-fixes, r=huonw

Drop is no longer a keyword, removed it from the intro docs.
Stop allocating view_items with @
De-managed ast::Path
De-manage Lifetime
De-share trait_ref

Also, makes the pretty-printer use & instead of @ as much as possible,
which will help with later changes, though in the interim has produced
some... interesting constructs.
De-manage OptVec<TyParamBounds>
De-share ast::Ty
Clean up warnings
Fix rustdoc and rusti
Fix broken tests
Fix merge-fallout-typo