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.
tutorial: stronger wording in build instructions
Test fixes and rebase conflicts from rollups

PRs closed as part of this:

Closes #12212 r=alexcrichton
Closes #12215 r=brson
Closes #12246 r=pcwalton
Closes #12247 r=cmr
Closes #12251 r=brson
Closes #12255 r=alexcrichton
Closes #12257 r=alexcrichton
Closes #12258 r=huonw
Closes #12259 r=huonw
Closes #12263 r=kballard
Closes #12269 r=alexcrichton
Update vim syntax for extern crate
Add CheckedDiv to vim syntax
auto merge of #12267 : alexcrichton/rust/rollup, r=alexcrichton

The last commit has the closed PRs
Update restrictions on rustdoc source rendering

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.
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
mk: Add some serious documentation and 'make help'

'make help' is implemented in a fairly ridiculous way, using awk
to parse comments out of Makefile.in and displaying them.
mk: Remove the concept of 'snapshot transitions'

This way of doing snapshots hasn't been used since 2011.
mk: Move most of Makefile.in to .mk files

Because the build system treats Makefile.in and the .mk files slightly
differently (.in is copied, .mk are included), this makes the system
more uniform. Fewer build system changes will require a complete
reconfigure.
mk: Add NO_MKFILE_DEPS for turning off rebuild from makefile changes
mk: Improve build system help commands
mk: Move version info to top of main.mk

Just so it's easier to find.
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.
mk: Address review feedback