introduce explaining variable refactoring.

[Vivek504]

Introduce Explaining Variable Refactoring

Reason: Long Statement

1. The length of the statement "final MethodHandle nonNullTest=lookup.findStatic(Objects.class,"nonNull",methodType(boolean.class,Object.class)).asType(methodType(boolean.class,cleanerClass));" is 160.
2. The length of the statement "final MethodHandle unmapper=filterReturnValue(directBufferCleanerMethod,guardWithTest(nonNullTest,cleanMethod,noop)).asType(methodType(void.class,ByteBuffer.class));" is 165.

Logic of the refactoring: Complicated expressions are replaced by explaining variables.