refactor(library/unifier): cleanup mk_macro_imitation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug in expand_rhs

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/converter): missing case

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): avoid exponential blowup when processing let-expressions with a lot of sharing, cleanup show-expression

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): remove mk_macro_imitation, we instead expand the macro before solving flex-rigid constraints

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