core, core/state, trie: EIP158, reprice & skip empty account write

This commit implements EIP158 part 1, 2, 3 & 4

1. If an account is empty it's no longer written to the trie. An empty
  account is defined as (balance=0, nonce=0, storage=0, code=0).
2. Delete an empty account if it's touched
3. An empty account is redefined as either non-existent or empty.
4. Zero value calls and zero value suicides no longer consume the 25k
  reation costs.

params: moved core/config to params

Signed-off-by: Jeffrey Wilcke <jeffrey@ethereum.org>

eth/downloader/downloader_test.go

@@ -109,7 +109,7 @@ func newTester() *downloadTester {
 // reassembly.
 func (dl *downloadTester) makeChain(n int, seed byte, parent *types.Block, parentReceipts types.Receipts, heavy bool) ([]common.Hash, map[common.Hash]*types.Header, map[common.Hash]*types.Block, map[common.Hash]types.Receipts) {
 	// Generate the block chain
-	blocks, receipts := core.GenerateChain(nil, parent, dl.peerDb, n, func(i int, block *core.BlockGen) {
+	blocks, receipts := core.GenerateChain(params.TestChainConfig, parent, dl.peerDb, n, func(i int, block *core.BlockGen) {
 		block.SetCoinbase(common.Address{seed})
 
 		// If a heavy chain is requested, delay blocks to raise difficulty