fee: use array for opcodes

Use less memory and have faster access.

name       old time/op  new time/op  delta
Opcode1-8  22.4ns ± 6%   3.0ns ± 6%  -86.63%  (p=0.000 n=10+10)

pkg/core/fee/opcode_test.go

@@ -0,0 +1,19 @@
+package fee
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
+)
+
+const feeFactor = 30
+
+// The most common Opcode() use case is to get price for single opcode.
+func BenchmarkOpcode1(t *testing.B) {
+	// Just so that we don't always test the same opcode.
+	script := []opcode.Opcode{opcode.NOP, opcode.ADD, opcode.SYSCALL, opcode.APPEND}
+	l := len(script)
+	for n := 0; n < t.N; n++ {
+		_ = Opcode(feeFactor, script[n%l])
+	}
+}