feat(kernel/metavar): collect unassigned metavariables while instantiating

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/type_checker): add type_checker::is_def_eq method that takes a justification as argument

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/converter): add temporary fix, we need to be able to backtrack constraints

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/unifier): add unify_fn skeleton

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

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

Conflicts:
	src/library/elaborator/elaborator.h