fix(frontends/lean/pp): bug in pp_abstraction_core

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/hop_match): make the higher order pattern matcher slightly stronger

It now can handle (?m t) where t is not a locally bound variable, but ?m and all free variables in t are assigned.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/simplifier): improve simplification by evaluation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/hop_match): style

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