Performance improvements for Kryo.isClosure()

[theigl]

This PR improves the default implementation of Kryo.isClosure() in two ways:

1. Check if the type is synthetic
2. Check if the simple name of the class contains a /

This is only slightly faster for hits, but significantly faster for misses:

Benchmark	Mode	Cnt	Score	Units
ClosureBenchmark.newHit	thrpt	2	231402690,531	ops/s
ClosureBenchmark.oldHit	thrpt	2	188889209,053	ops/s
ClosureBenchmark.newMiss	thrpt	2	1796295832,164	ops/s
ClosureBenchmark.oldMiss	thrpt	2	174581347,713	ops/s

I hope that Class.isSynthetic() is true for closures on all JDKs.