refactor(*): uses aliases for unordered_map and unordered_set

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): change function application precedence

Now, we can write

  Pi (x y : A), R x y -> R y x

instead of

  Pi (x y : A), (R x y) -> (R y x)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lean): remove frontend class, it is not needed anymore

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): Scopes in the default Lean frontend

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