Only create one UnsafeAllocator instance

[kb-1000]

This reduces unnecessary allocations, and may potentially even help the JIT (at least for regular JVMs, unsure about Android) optimize the code a little bit more.

[eamonnmcmanus]

Thanks, I agree this seems like a good idea. One small thing: this line

    final String exceptionMessage = UnsafeAllocator.checkInstantiable(rawType);

causes the static initializer for UnsafeAllocator to run and that will now attempt to create an UnsafeAllocator. Could you wrap the code-block containing the above line in if (useJdkUnsafe)?

[Marcono1234]

Could you wrap the code-block containing the above line in if (useJdkUnsafe)?

Maybe it would be better to move the method checkInstantiable to ConstructorConstructor instead? The point of this check is to also provide a correct exception message if useJdkUnsafe == false, see comment above the checkInstantiable call.