feat(builtin/hep): replace hallext axiom with theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/simplifier): bug in heterogeneous equality support, and universe commutativity support in the simplifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): add new simplifier test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/simplifier): missing check in mk_hcongr_th

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): bug in 'using' construct

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