Provide API to run formal verification programmatically.

[marcogelmi]

It would be useful to have an API that we can call to verify that two implementations are equivalent. This would be essentially the same thing that the check_ir_equivalence_main tool does.

This API would take two xls::Function* and return a StatusOr and possibly an opaque error message for debugging purposes (e.g. variable assignments, same as the command line tool).