Implement HTMLScriptElement.text
Improve Option deconstruction
Update to latest rust-layers

This should reduce memory usage along layer boundaries.
Remove the Rust submodule.
Move most of Makefile.in to mk/main.mk

This allows modifying it without re-running configure.
check the type/language of script elements
Allow marking tests as flaky by target OS.
Add a (broken) Acid2 reftest. Fix #1993

The rendering is incorrect
and the test is marked as flaky on Linux or on GPU rendering,
but this should help catch further regressions.

The test is modified to add the following CSS:

    .intro { display: none }
    html #top { margin-top: 0 }

To disable the "scrolling" part of the test.

The reference is modified to match the current rendering of the test:

* Rows 4 and 5 (eyes) have a red background.
* The nose is not quite where it should be.
* Row 12’s height is 3px more than it should be.

(See http://www.webstandards.org/action/acid2/guide/ for row numbers.)

The nose issue seems be related to rounding vs truncating
when pixel-snapping borders, but this is only a guess.
Merge pull request #3129 from mrobinson/layer-edges

Update to latest rust-layers
Merge pull request #2741 from SimonSapin/acid2-reftest

Add a (broken) Acid2 reftest. Fix #1993
Merge pull request #3125 from seanmonstar/script-type

Check the type/language of script elements (fixes #2828); r=SimonSapin+Ms2ger
Update web-platform-tests.
Move rust-snapshot-hash to the root.
Merge pull request #3095 from ChrisParis/HTMLScriptElement.text

Implement HTMLScriptElement.text; r=Ms2ger
Merge pull request #3132 from Ms2ger/update-wpt

Update web-platform-tests; r=jgraham
Merge pull request #3130 from SimonSapin/remove-rust-submodule

Remove rust submodule