events: version the MessageReceipt structure.

This is implemented as an internal version discriminator. An alternative
would've been to turn MessageReceipt into an interface with two backing
structs for v0 and v1. But that approach would've broken all usage sites.

CBOR serde for MessageReceipt is no longer automatically generated. We
use custom logic to deal with versioning in both directions. This logic
uses the cborgen serde code as a basis.

chain/types/message_receipt.go

@@ -3,19 +3,52 @@ package types
 import (
 	"bytes"
 
+	"github.com/filecoin-project/go-state-types/exitcode"
 	"github.com/ipfs/go-cid"
+)
 
-	"github.com/filecoin-project/go-state-types/exitcode"
+type MessageReceiptVersion byte
+
+const (
+	// MessageReceiptVersion0 refers to pre FIP-0049 receipts.
+	MessageReceiptVersion0 MessageReceiptVersion = 0
+	// MessageReceiptVersion1 refers to post FIP-0049 receipts.
+	MessageReceiptVersion1 MessageReceiptVersion = 1
 )
 
 type MessageReceipt struct {
+	version MessageReceiptVersion
+
 	ExitCode   exitcode.ExitCode
 	Return     []byte
 	GasUsed    int64
 	EventsRoot *cid.Cid // Root of Event AMT
 }
 
+// NewMessageReceiptV0 creates a new pre FIP-0049 receipt with no capability to
+// convey events.
+func NewMessageReceiptV0(exitcode exitcode.ExitCode, ret []byte, gasUsed int64) MessageReceipt {
+	return MessageReceipt{
+		version:  MessageReceiptVersion0,
+		ExitCode: exitcode,
+		Return:   ret,
+		GasUsed:  gasUsed,
+	}
+}
+
+// NewMessageReceiptV1 creates a new pre FIP-0049 receipt with the ability to
+// convey events.
+func NewMessageReceiptV1(exitcode exitcode.ExitCode, ret []byte, gasUsed int64, eventsRoot *cid.Cid) MessageReceipt {
+	return MessageReceipt{
+		version:    MessageReceiptVersion1,
+		ExitCode:   exitcode,
+		Return:     ret,
+		GasUsed:    gasUsed,
+		EventsRoot: eventsRoot,
+	}
+}
+
 func (mr *MessageReceipt) Equals(o *MessageReceipt) bool {
-	return mr.ExitCode == o.ExitCode && bytes.Equal(mr.Return, o.Return) && mr.GasUsed == o.GasUsed &&
+	return mr.version == mr.version && mr.ExitCode == o.ExitCode && bytes.Equal(mr.Return, o.Return) && mr.GasUsed == o.GasUsed &&
 		(mr.EventsRoot == o.EventsRoot || (mr.EventsRoot != nil && o.EventsRoot != nil && *mr.EventsRoot == *o.EventsRoot))
 }