Stable sort for (data/newtype) instances

haddock-api/src/Haddock/Interface/AttachInstances.hs

html-test/ref/Bug1004.html

html-test/ref/Bug1103.html

html-test/ref/Bug548.html

html-test/ref/HiddenInstances.html

html-test/ref/Instances.html

html-test/ref/SpuriousSuperclassConstraints.html

html-test/ref/TypeFamilies.html

html-test/ref/TypeFamilies2.html

html-test/ref/TypeFamilies3.html

hypsrc-test/ref/src/Classes.html

hypsrc-test/ref/src/Records.html

latex-test/ref/TypeFamilies3/TypeFamilies3.tex