Hyperlinks and `values_first_mode`

PrintBox-only features: Optionally emit source code pointers as hyperlinks; values_first_mode which puts the result of a computation in the header, instead of the source code location of the computation. It's important to use values_first_mode when using hyperlinks, otherwise the tree nodes will not be foldable. Also, more compact printing of small sexp values (inline with the variable / function name).