Start using more typed units in the compositor

Now that rust-layers is starting to support typed units, we can use
them more thoroughly in the compositor. This is a step toward removing
as many untyped units as possible.
Fix task failure when loading wikipedia.

There is still an issue with alpha display, so there is
likely a problem elsewhere, but this at least allows the
page to load for now (ref #2554).
Move the script type and language checks into HTMLScriptElement.

This is IMO a more sensible place for them to live, and prepares for the
demise of hubbub_html_parser.rs, as we move to html5ever.
Merge pull request #3196 from Ms2ger/script

Move the script type and language checks into HTMLScriptElement.
Merge pull request #3173 from mrobinson/typed-units-1

Start using more typed units in the compositor
Add support for borders on inline elements.

Fix ahem reftests on mac, by disabling text AA in reftest mode.

Also fix precision issues in font metrics to correct height
and baseline calculations.
Merge pull request #3195 from glennw/image-format

Fix task failure when loading wikipedia.
Make some of Node's members private.
Merge pull request #3182 from glennw/inline-text-border

Add support for borders on inline elements
Merge pull request #3202 from Ms2ger/node-priv-members

Make some of Node's members private; r=Manishearth
Dump initial prototype of devtools server into the build. Expect lies if you try to use it for anything real.
Remove broken AnyRefExt implementation for devtools actors.
Add devtools_traits crate and create infrastructure for actual communication between server and script task.
Trailing ws.
Enable executing JS snippets in the context of the main Servo window and viewing the responses from the Firefox remote console.
Split devtools implementation into sensible modules.
Documentation and TODOs.
Suppress unnecessary mutable warning.
Revert submodule change.
Avoid selecting on the devtools port if no devtools server exists.