Add @Spell clusters to Vim syntax file

Add @Spell clusters to Vim syntax highlighting file to do spell checking
only inside comments and strings
Introduced a chapter that describes how to perform callbacks from C to Rust
Further details on channel idea
Support linking of rlib files in rustpkg
Fix indention
Update guide-ffi.md

Simplified the first examples to demonstrate callbacks without other threads involved and shortened the elaboration about async callbacks.
doc: Boxes diagram shows enum value inside box.
doc: add note below diagram about memory layout.
Fixed bug when initialising bitv from bool=true
syntax::ext: replace span_fatal with span_err in many places.

This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
doc: fix version stamp for TeX files

This fixes a regression introduced in
3d57b240ab1fd038b3a41fe2042dc9a0544ea53e.
Elaborate manual on matching (dereference patterns, lvalue/rvalue matching)

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
doc: fix rust.md fallout
Fixed typos in comments of librustc backend.
rustc: Feature gate `log_syntax!`. Closes #11602
handle zero-size allocations correctly

The `malloc` family of functions may return a null pointer for a
zero-size allocation, which should not be interpreted as an
out-of-memory error.

If the implementation does not return a null pointer, then handling
this will result in memory savings for zero-size types.

This also switches some code to `malloc_raw` in order to maintain a
centralized point for handling out-of-memory in `rt::global_heap`.

Closes #11634
Actually force a / in the path for ext crates

If the library is in the working directory, its path won't have a "/"
which will cause dlopen to search /usr/lib etc. It turns out that Path
auto-normalizes during joins so Path::new(".").join(path) is actually a
no-op.
Make WindowsPath::new("C:foo").root_path() return Some("C:")
doc: fix dangling links in rust.md

Fixes #11559 (not that the manual couldn't use a review).
auto merge of #11619 : adridu59/rust/patch-md, r=cmr

Noticeably necroes #10892.
Also closes #11559.

r? @alexcrichton