Empty list pattern was not handled correctly.
Fixed various return/yields with composite argument

The muRascal does not treat yield/return well when there is a composite
argument, e.g. return len + pos is treated as (return len) + pos.

Ultimately we should change the syntax. Now I have added parentheses
everywhere.
Replaced a size_set by size_mset
Non-linear set patterns are now handled correctly
Non-linear set patterns with anonymous variables now handled correctly
Fixed issue in testreport_add
Christmas present: 81% of tests pass.