core, core/vm: added gas price variance table

This implements 1b & 1c of EIP150 by adding a new GasTable which must be
returned from the RuleSet config method. This table is used to determine
the gas prices for the current epoch.

Please note that when the CreateBySuicide gas price is set it is assumed
that we're in the new epoch phase.

In addition this PR will serve as temporary basis while refactorisation
in being done in the EVM64 PR, which will substentially overhaul the gas
price code.

core/config.go

@@ -21,6 +21,7 @@ import (
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/core/vm"
+	"github.com/ethereum/go-ethereum/params"
 )
 
 var ChainConfigNotFoundErr = errors.New("ChainConfig not found") // general config not found error
@@ -35,6 +36,8 @@ type ChainConfig struct {
 	DAOForkBlock   *big.Int `json:"daoForkBlock"`   // TheDAO hard-fork switch block (nil = no fork)
 	DAOForkSupport bool     `json:"daoForkSupport"` // Whether the nodes supports or opposes the DAO hard-fork
 
+	HomesteadGasRepriceBlock *big.Int `json:"homesteadGasRepriceBlock"` // Homestead gas reprice switch block (nil = no fork)
+
 	VmConfig vm.Config `json:"-"`
 }
 
@@ -45,3 +48,14 @@ func (c *ChainConfig) IsHomestead(num *big.Int) bool {
 	}
 	return num.Cmp(c.HomesteadBlock) >= 0
 }
+
+// GasTable returns the gas table corresponding to the current phase (homestead or homestead reprice).
+//
+// The returned GasTable's fields shouldn't, under any circumstances, be changed.
+func (c *ChainConfig) GasTable(num *big.Int) params.GasTable {
+	if c.HomesteadGasRepriceBlock == nil || num == nil || num.Cmp(c.HomesteadGasRepriceBlock) < 0 {
+		return params.GasTableHomestead
+	}
+
+	return params.GasTableHomesteadGasRepriceFork
+}