core/types: make blocks immutable

core/block_cache_test.go

core/block_processor.go

core/block_processor_test.go

core/chain_makers.go

core/chain_manager.go

core/chain_manager_test.go

core/genesis.go

core/state_transition.go

core/transaction_pool.go

core/types/block.go

core/vm_env.go

eth/downloader/downloader_test.go

miner/agent.go

miner/remote_agent.go

miner/worker.go

tests/block_test_util.go

xeth/xeth.go