Make Attr::owner immutable.

Nobody needs to change the element it's associated with, so there's no reason
to use a Cell here.
Merge pull request #3031 from Ms2ger/attr-cell

Make Attr::owner immutable; r=jdm
Add TreeWalker blob.
Add DOM Range blob.
Add DOM NodeIterator blob (include DOM NodeFilter interface).
Merge pull request #2987 from saneyuki/idl

Add some blobs for DOM Bindings
Prefer *Derived helper functions (is_*element) than string comparison
Merge pull request #3053 from brunoabinader/use-is-element-helpers

Prefer *Derived helper functions (is_*element) than string comparison