Remove fields index and value from Node

This way, Bundles may contain fields named index and value.
Fix very long compile times for init_mapping_table

The culprit was repeated construction/destruction of std::strings.
Instead, pass const char*s through the constructors until the std::string
truly does need to be constructed.
Speed up VCD compilation and dumping

Split VCD dump into multiple functions; get rid of strlen call; use much
shorter variable names in output