Make diff only on results

[vch9]

Diff should be computed only on function results.
Related to #9, parameters will always be equal in prev/next snapshots.

The programmer could introduce a new printer to better debug on errors.
However, printing a parameter should not create a diff.

When the diff will be displayed, function + parameters + result will be printed
with given printers. But internally, errors will only be on results.

[vch9]

Adding a printer will not change anything. The new printer will be used to show the previous snapshot. The diff could never comes from parameters.