#72 Reduce compile time & debug symbols size by getting rid of compile-time polymorphism

Template nodes are too heavy, slowing down compile times and inflating debug symbols size (~170M for libnodes.so). This can be reduced by replacing static polymorphism with dynamic, which we already have anyway. There are two possible issues, though: - any use of dynamic casts to BaseValue will have to be replaced with type checks - possible performance downsides (current measurements don't show it, but they aren't accurate enough to be sure)