Create a new instance when setting big.Int fields

That is, do not reuse the existing one, because it may be shared by
multiple header instances.  The goal here is to make it safe to
value-copy header structs.

block/header.go

@@ -162,7 +162,7 @@ func (h *Header) Number() *big.Int {
 //
 // It stores a copy; the caller may freely modify the original.
 func (h *Header) SetNumber(newNumber *big.Int) {
-	h.fields.Number.Set(newNumber)
+	h.fields.Number = new(big.Int).Set(newNumber)
 }
 
 // GasLimit is the gas limit for transactions in this block.
@@ -196,7 +196,7 @@ func (h *Header) Time() *big.Int {
 //
 // It stores a copy; the caller may freely modify the original.
 func (h *Header) SetTime(newTime *big.Int) {
-	h.fields.Time.Set(newTime)
+	h.fields.Time = new(big.Int).Set(newTime)
 }
 
 // Extra is the extra data field of this block.
@@ -240,7 +240,7 @@ func (h *Header) ViewID() *big.Int {
 //
 // It stores a copy; the caller may freely modify the original.
 func (h *Header) SetViewID(newViewID *big.Int) {
-	h.fields.ViewID.Set(newViewID)
+	h.fields.ViewID = new(big.Int).Set(newViewID)
 }
 
 // Epoch is the epoch number of this block.
@@ -254,7 +254,7 @@ func (h *Header) Epoch() *big.Int {
 //
 // It stores a copy; the caller may freely modify the original.
 func (h *Header) SetEpoch(newEpoch *big.Int) {
-	h.fields.Epoch.Set(newEpoch)
+	h.fields.Epoch = new(big.Int).Set(newEpoch)
 }
 
 // ShardID is the shard ID to which this block belongs.