This change rearranges the accessor methods in block.go and fixes some minor issues with
the copy-on-write logic of block data. Fixed issues:

- Block.WithWithdrawals did not create a shallow copy of the block.

- Block.WithBody copied the header unnecessarily, and did not preserve withdrawals.

However, the bugs did not affect any code in go-ethereum because blocks are *always*
created using NewBlockWithHeader().WithBody().WithWithdrawals()