Performance issues related to FieldPredicates class

[dasteg]

The method FieldPredicates.named(string) has a major performance issue. The predicate compiles the name to regex pattern inside the test method. This makes it extremely slow when creating a large number of randomized objects.

Pattern is immutable and thread-safe and can be compiled before creating the predicate. This reduces execution time a lot and also saves quite some memory garbage.

created a pull request #367