feat(library/standard): add decidability of le for int
feat(library/standard/sum.lean): add properties of sum
feat(library/standard/hott/fibrant.lean): add fibrant to library
fix(library/standard/data/int): remove misnamed file
chore(library/standard/data/sum): remove hints, they are not needed after the fix

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>