Synthesis and rewriter tests

[dorchard]

Some of these look like they might be failing still.

[jackohughes]

The two failing synthesiser tests graded-base/either.gr and graded-base/drop.gr are not failing because of outdated golden outputs. They are due to issues with the actual synthesiser/constraints themselves. drop.gr is producing a program which doesn't type check and either.gr is timing out. Looking into both now.