Check for unexpected instances of boxing

We use the AnyVal types of Maybe (and specialized MaybeFoo types) and DataValue to avoid the need to box objects. However, there are times when these get boxed unexpectedly. We should look through and make sure non of this boxing happens at a time when we care if it gets boxed or not.

To check, we can dissasemble the compiled code:

javap -c -l `find . -name *.class | grep scala-2.12` > src.bytecode

then search the resulting assembly for strings along the lines of DataValue."<init>", which would indicate boxing.