tlv: make TlvType method for TlvType interface public

With this change, callers can now examine a struct instance of the
`TlvType` instance to figure out programmatically which integer type is
maps to. This'll be useful when decoding optional TLV types into a wire
struct (knowing when to set it to Some vs None).

tlv/internal/gen/gen_tlv_types.go

@@ -20,10 +20,12 @@ package tlv
 
 type tlvType{{ $index }} struct{}
 
-func (t *tlvType{{ $index }}) typeVal() Type {
+func (t *tlvType{{ $index }}) TypeVal() Type {
 	return {{ $index }}
 }
 
+func (t *tlvType{{ $index }}) tlv() {}
+
 type TlvType{{ $index }} = *tlvType{{ $index }}
 {{- end }}
 `

tlv/record_type.go

@@ -62,7 +62,7 @@ func (t *RecordT[T, V]) Record() Record {
 	tlvRecord, ok := any(&t.Val).(RecordProducer)
 	if !ok {
 		return MakePrimitiveRecord(
-			t.recordType.typeVal(), &t.Val,
+			t.recordType.TypeVal(), &t.Val,
 		)
 	}
 
@@ -72,7 +72,7 @@ func (t *RecordT[T, V]) Record() Record {
 
 	return Record{
 		value:      ogRecord.value,
-		typ:        t.recordType.typeVal(),
+		typ:        t.recordType.TypeVal(),
 		staticSize: ogRecord.staticSize,
 		sizeFunc:   ogRecord.sizeFunc,
 		encoder:    ogRecord.encoder,

tlv/tlv_type_param.go

@@ -5,7 +5,13 @@ import "fmt"
 // TlvType is an interface used to enable binding the integer type of a TLV
 // record to the type at compile time.
 type TlvType interface {
-	typeVal() Type
+	// TypeVal returns the integer TLV type that this TlvType struct
+	// instance maps to.
+	TypeVal() Type
+
+	// tlv is an internal method to make this a "sealed" interface, meaning
+	// only this package can declare new instances.
+	tlv()
 }
 
 //go:generate go run internal/gen/gen_tlv_types.go -o tlv_types_generated.go