feat(frontends/lean): add 'show' expression syntax sugar

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): new 'have' expression

Add 'have' notation suggested by Jeremy Avigad.
Add his example to the test suite.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): new 'have' expression