Given a sequence and a structure, evaluate the energy.
TODO switch to 'SSTree [Energy]' type!
Evaluate the energy of a secondary structure tree with sequence. We abuse the normal folding functions with a dummy table full of (one :: Energy). This is probably slower than another method but quickly written.
TODO this is basically crapfuck ;-) Should use the FoldFunctions directly instead of that strange table. Should not xxxOpt either.