container.info handling used to be handled by the the falco_outputs
object. However, this caused problems for applications that only used
the falco engine, doing their own output formatting for matching events.

Fix this by moving output formatting into the falco engine itself. The
part that replaces %container.info/adds extra formatting to the end of a
rule's output now happens while loading the rule.

Validate rule outputs when loading rules by attempting to create a
formatter based on the rule's output field. If there's an error, it will
propagate up through load_rules and cause falco to exit rather than
discover the problem only when trying to format the event and the rule's
output field.

This required moving formats.{cpp,h} into the falco engine directory
from the falco general directory. Note that these functions are loaded
twice in the two lua states used by falco (engine and outputs).

There's also a couple of minor cleanups:

 - falco_formats had a private instance variable that was unused, remove
   it.
 - rename the package for the falco_formats functions to formats instead
   of falco so it's more standalone.
 - don't throw a c++ exception in falco_formats::formatter. Instead
   generate a lua error, which is handled more cleanly.
 - free_formatter doesn't return any values, so set the return value of
   the function to 0.

Add the ability to check falco's return code with exit_status and to
generally match stderr with stderr_contains in a test.

Use those to create a test that has an invalid output expression using
%not_a_real_field. It expects falco to exit with 1 and the output to
contain a message about the invalid output.

Change falco_engine::process_event to return a unique_ptr that wraps the
rule result, so it won't be leaked if this method throws an exception.

This means that callers don't need to create their own.