Implement flip flop operator

[richardboehme]

TIL this existed :^)

We now also pass the whole language/if_spec!

This whole thing uses some interesting hacks, where I'm not sure if they are nice:

• I reuse the ElseInstruction to fetch the instructions of the two conditions (taken from IfInstruction)
• I hoist the flip flop states to the top of the generated C++ file, not sure if this is nice but we need to store the state somewhere...

[herwinw]

This was one of the operators that I wanted to add to #2310 as "we are never going to support this, so maybe we should still be able to run those specs" 😅

[herwinw]

I added a commit to enable the specs in precedence_spec.rb as well, since these now work too.