core: Remove use of deprecated `drop`
syntax: remove parsing destructors
rustc: remove the rest of drop

Removes:

ast::struct_def::dtor
syntax::ast::ii_dtor
syntax::visit::fk_dtor
syntax::ast_map::node_dtor
syntax:struct_dtor
allow parsing attributes on struct fields
syntax: remove parse::token::{dtor,literally_dtor}
remove some warnings
rustdoc: Remove a now invalid test
pipes: use finally to fix pipes::try_recv
auto merge of #6148 : erickt/rust/remove-drop, r=pcwalton

The drop block has been deprecated for quite some time. This patch series removes support for parsing it and all the related machinery that made drop work.

As a side feature of all this, I also added the ability to annote fields in structs. This allows comments to be properly associated with an individual field. However, I didn't update `rustdoc` to integrate these comment blocks into the documentation it generates.