add pretty printing of 'lookupswitch' instruction'.

Also some refactoring that makes Diff.formatChanges for readable.
another step towards proper label merging.

We now filter out all input labels that are not jump targets (they
are useless for us), and support label-aware diffing for both
jump instructions and lookupswitch.

Still remains the main display issue: there is currently no way
to know when looking at the output if a target ins. number
references the first file or the second file. Results are
sometime surprising, but it only affects the display, not whether
two instruction are different or not.