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>

core/vm/environment.go

@@ -23,20 +23,11 @@ import (
 	"github.com/ethereum/go-ethereum/params"
 )
 
-// RuleSet is an interface that defines the current rule set during the
-// execution of the EVM instructions (e.g. whether it's homestead)
-type RuleSet interface {
-	IsHomestead(*big.Int) bool
-	// GasTable returns the gas prices for this phase, which is based on
-	// block number passed in.
-	GasTable(*big.Int) params.GasTable
-}
-
 // Environment is an EVM requirement and helper which allows access to outside
 // information such as states.
 type Environment interface {
 	// The current ruleset
-	RuleSet() RuleSet
+	ChainConfig() *params.ChainConfig
 	// The state database
 	Db() Database
 	// Creates a restorable snapshot
@@ -115,6 +106,9 @@ type Database interface {
 	// Exist reports whether the given account exists in state.
 	// Notably this should also return true for suicided accounts.
 	Exist(common.Address) bool
+	// Empty returns whether the given account is empty. Empty
+	// is defined according to EIP161 (balance = nonce = code = 0).
+	Empty(common.Address) bool
 }
 
 // Account represents a contract or basic ethereum account.