Force infallible binding methods to return void.(fixes #1933)
Rename squirrel_away_unboxed to squirrel_away_unique.(fixes #1938)
auto merge of #1944 : lpy/servo/issue1938, r=Ms2ger

see #1938
auto merge of #1945 : Ms2ger/servo/1196-isDefinitelyObject, r=jdm

... and fix nearby whitespace (fixes #1196).
auto merge of #1943 : lpy/servo/issue1933, r=jdm

see #1933
Split TCast::to into TCast::to_unchecked and TCast::to.
auto merge of #1864 : saneyuki/servo/split_cast_to, r=jdm

fix #1836
Move attributes-related functions onto JS<Element>.