rustdoc: Strip impls of stripped private types

In strip-private, also strip impls of traits for private types. This
fixes the search index so searching for "drop", "eq", etc doesn't throw
an exception.
rustdoc: Strip impls of traits on #[doc(hidden)] types

In the strip-hidden pass, record all types that were stripped, and make
a second pass stripping all impls of traits for these types.
Update vim syntax for extern crate
Add CheckedDiv to vim syntax
auto merge of #12195 : kballard/rust/rustdoc-strip-impls-of-stripped, r=cmr

Strip trait impls for types that are stripped either due to the strip-hidden or strip-private passes.

This fixes the search index including trait methods on stripped structs (which breaks searching), and it also removes private types from the implementors list of a trait.

Fixes #9981 and #11439.
Add crate to emacs and kate modefiles
Add crate keyword to gedit language spec
auto merge of #12271 : kballard/rust/vim-extern-crate, r=huonw
auto merge of #12277 : alexcrichton/rust/fix-rustdoc-render, r=huonw

The std macros used to be injected with a filename of "<std-macros>", but macros
are now injected with a filename of "<{} macros>" where `{}` is filled in with
the crate name. This updates rustdoc to understand this new system so it'll
render source more frequently.
Register new snapshots

This enables the parser error for `extern mod` => `extern crate` transitions.
extern mod => extern crate

This was previously implemented, and it just needed a snapshot to go through