Rather than following the spec exactly and creating lowercase strings,
we can simply do a case-insensitive string comparison. The caveat is
that creating attributes must follow the spec by creating the attribute
name with a lowercase string.
This is similar to how Gecko avoids a reference cycle, where both the
NamedNodeMap and Attribute would otherwise store a strong reference to
their associated Element. Gecko manually clears stored raw references
when an Element is destroyed, whereas we use weak references to do so
automatically.
Attribute's ownerElement getter and setter are moved out of line to
avoid an #include cycle between Element and Attribute.
