Experiment: Move OP_CALL code outside main loop
Rolled back change to RVM: local variables -> instance variables

In commit b9af615 I had replaced the local variables of the RVM method
executeProgram by instance variables and did observe (and even
documented in Measurements.txt) a significant speed-up.

In subsequent commits it turned out that compiled code had become 5
times slower. After an extensive analysis it turns out that the above
change was the culprit.

Current situation: compiled code is back to being 12.5 x faster than
interpreted code (as opposed to the fabulous 22 x reported earlier).

It remains a mystery how these much too optimistic measurements could
occur. Conjecture: code outside the compiler (potential candidate:
string origins) made the interpreter temporarily much slower. 

Disputed measurement 211492 (compiled) 1663085 (interpreted)
This measurement     202251 (compiled)  982352 (interpreted)

This confirms that the speed of the compiled code was roughly the same,
but that the interpreter was much slower.
Fixed error in value_to_string
Added two cases to backtrackfree
Progress with testing
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.
Merge branches 'master' and 'muAnd-muOr' of https://github.com/cwi-swat/rascal into muAnd-muOr