[SPARK-6030] [CORE] Using simulated field layout method to compute class shellSize

SizeEstimator gives wrong result for Integer on 64bit JVM with UseCompressedOops on, this pr fixes that. For more details, please refer [SPARK-6030](https://issues.apache.org/jira/browse/SPARK-6030)
sryza, I noticed there is a pr to expose SizeEstimator, maybe that should be waited by this pr get merged if we confirm this problem.
And shivaram would you mind to review this pr since you contribute related code. Also cc to srowen and mateiz

Author: Ye Xianjin <advancedxy@gmail.com>

Closes #4783 from advancedxy/SPARK-6030 and squashes the following commits:

c4dcb41 [Ye Xianjin] Add super.beforeEach in the beforeEach method to make the trait stackable.. Remove useless leading whitespace.
3f80640 [Ye Xianjin] The size of Integer class changes from 24 to 16 on a 64-bit JVM with -UseCompressedOops flag on after the fix. I don't how 100000 was originally calculated, It looks like 100000 is the magic number which makes sure spilling. Because of the size change, It fails because there is no spilling at all. Change the number to a slightly larger number fixes that.
e849d2d [Ye Xianjin] Merge two shellSize assignments into one. Add some explanation to alignSizeUp method.
85a0b51 [Ye Xianjin] Fix typos and update wording in comments. Using alignSizeUp to compute alignSize.
d27eb77 [Ye Xianjin] Add some detailed comments in the code. Add some test cases. It's very difficult to design test cases as the final object alignment will hide a lot of filed layout details if we just considering the whole size.
842aed1 [Ye Xianjin] primitiveSize(cls) can just return Int. Use a simplified class field layout method to calculate class instance size. Will add more documents and test cases. Add a new alignSizeUp function which uses bitwise operators to speedup.
62e8ab4 [Ye Xianjin] Don't alignSize for objects' shellSize, alignSize when added to state.size. Add some primitive wrapper objects size tests.

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