Add statistics dumper for C tests

[MattWindsor91]

This would output a lot of things that aren't included in a Litmus header, but are useful for making decisions such as how to lift the test to a harness.

This would basically just be a list of category-count pairs like so:

threads 2
returns 10
literal-bools 90

These stats would be useful because:

• thread count can be used for rejecting over-cap tests;
• return count can be used for determining whether to pass -ascall to litmus;
• literal bool count can be used for determining whether to splice stdbool includes.

Eventually there'd be a non-threads count that can be used to determine whether to activate indirect mode.

[MattWindsor91]

Done, but could be expanded.