fix(library/unifier): bug in occurs_check, the dependency may be eliminated by reducing term

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library/standard/data/nat/div): remove TODO

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): 'let [inline]' is the default

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/standard): define abbreviations using '@'

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