This patch is really two separate changes.

The first change is that rust crates are large, standalone entities that
may contain multitudes of source files.  It therefore doesn't make sense
to keep them in SOURCES, as we have been doing.  Moving to use cargo
will require a higher-level approach, which suggests that we need a
different, higher-level representation for Rust sources in the build
system.

The representation here is to have the build system refer to things
defined in Cargo.toml files as the entities dealt with in the build
system, and let Cargo deal with the details of actually building things.
This approach means that adding a new crate to an existing library just
requires editing Rust and Cargo.toml files, rather than dealing with
moz.build, which seems more natural to Rust programmers.  By having the
source files for libraries (and binaries in subsequent iterations of
this support) checked in to the tree, we can also take advantage of
Cargo.lock files.

The second is that we switch the core build system over to building via
cargo, rather than invoking rustc directly.

We also clean up a number of leftover things from the Old Way of doing
things.  A number of tests are added to confirm that we'll only permit
crates to be built that have dependencies in-tree.

We are going to need a Cargo.toml for all Rust code in the tree, no
matter how trivial.

We need to parse Cargo.toml files from moz.build to determine crate
package names.

rlibs are going to be less important once we start building with cargo:
the focus will move to crates instead, so that's what we should call the
moz.build object.

We've going to use it, the location of it should be configurable.

Through an oversight, we listed librul.a twice when linking libxul: once
as part of the "objects" we were linking, and once as a static library.
This duplication is unnecessary and would cause problems later when we
try to generate librul.a via cargo, as cargo will put it someplace
different from where we expect and the two names will conflict.  Let's
have rules.mk be the single source of truth for how librul.a is named,
and then the code to link libxul can simply refer to that name.

Bug 1292505 (Part 2) - Remove remaining references to Downscaler in the SurfacePipe decoders. r=edwin

Bug 1292505 (Part 1c) - Use ExplicitOutputSize() instead of Downscaler::TargetSize() in nsICODecoder. r=edwin

Bug 1292505 (Part 1b) - Update DecoderFactory to use SetOutputSize(), and propagate the changes to RasterImage. r=edwin

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Backed out changeset 72e5792fe990 (bug 1286041)
Backed out changeset 14d4b1e011ad (bug 1286041)

--HG--
extra : rebase_source : 1db28655abbb337e537a67d09e2af243ae94f081

By using nsCOMArray::Forget, we can pass the references we hold in an
nsCOMArray directly to the XPCOM caller, without having to go through an
extra cycle of AddRef/Release calls.  This way should be slightly faster
and it also results in smaller code.  Notice that we don't have to worry
about removing the OOM check in the places that use Forget(), because we
were already using moz_xmalloc to allocate the outparam array, and
moz_xmalloc aborts the process on OOM, just as Forget() does.

The nsNavHistoryFolderResultNode::GetQueries change is thrown in for fun.

MozReview-Commit-ID: KGAS6EhnwAI

Bug 1278135 - Link against less of Clang and LLVM statically when building the clang plugin, r=glandium

--HG--
extra : rebase_source : dc96a8d1b055531e3d6129b886597ea00f84fa43

Bug 1291064 - Make js::HelperThread use js::ThisThread::SetName instead of PR_SetCurrentThreadName; r=terrence

Bug 1290116 P6 Add a wpt test case verifying getRegistrations() does not return registrations with a pending uninstall. r=asuth

Bug 1290116 P5 Make navigator.serviceWorker.getRegistrations() ignore uninstalling registrations. r=asuth

Bug 1285271 - Disable_copy_on_write_canvas_on_windows_r_jnicol: Disable copy-on-write canvas on Windows. r=jnicol

Bug 1287411 - Ensure we have enough ballast space in jit::PropertyReadOnPrototypeNeedsTypeBarrier. r=jandem