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 }}
 `