Fix naming of diff png files when multiple reftest lists in use. Import a few css21 reftests.
Remove content tests that duplicate wpt tests.

In particular, the test coverage is provided by:

dom/nodes/Node-childNodes.html
dom/nodes/Document-createElementNS.html
dom/historical.html
dom/nodes/Document-createProcessingInstruction.html
dom/nodes/Document-doctype.html
dom/nodes/Document-importNode.html
dom/nodes/Document-createElement.html
dom/nodes/Document-getElementsByTagName.html
Use Attr::value() whenever possible
Make AttrValue::as_slice impl from Str
Merge pull request #3124 from brunoabinader/attr-changes

Attr, AttrValue changes; r=Ms2ger
Merge pull request #3133 from Ms2ger/rm-contenttests

Remove content tests that duplicate wpt tests; r=abinader
Implement cache for remaining Document live collections
Update the description about Rust compiler which we uses.
Merge pull request #3136 from saneyuki/readme

Update the description about Rust compiler which we uses.
Merge pull request #3090 from brunoabinader/document-cached-live-htmlcollections

Implement cache for remaining Document live collections; r=Ms2ger
Merge pull request #3128 from glennw/add-ref-tests

Fix naming of diff png files when multiple reftest lists in use. Import a few css21 reftests.
Temporarily revert rust-layers, as the most recent change broke the acid2 test on mac/travis.
Merge pull request #3143 from glennw/revert-layers

Temporarily revert rust-layers, as the most recent change broke the acid2 test on mac/travis.