fix(library/hott) fix funext.lean by making funext an instance
chore(library/hott) move theorem about precomposition to its own file
feat(library/hott) add theorem: assuming function extensionality, precomposing and postcomposing of equivalences is an equivalence
feat(library/hott) add theorem: assuming function extensionality, precomposing and postcomposing of equivalences is an equivalence