feat(builtin/tactic): add the 'skip' (bogus) tactic for ignoring a proof hole in a big proof

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): try first projection before imitation in the higher-order unifier

Projections build more general solutions. This commit also adds a test that demonstrates the issue. Before this commit, the elaborator would produce the "constant" predicate (fun x, a + b = b + a).

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