[release/1.4.18] 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.

(cherry picked from commit 64af2aafda)

core/vm/environment.go

@@ -20,12 +20,16 @@ import (
 	"math/big"
 
 	"github.com/ethereum/go-ethereum/common"
+	"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