Make field writing order consistent between CODE_SIZE and SPEED

[jcready]

Fixes #453

Binary serialization is explicitly not expected to be consistent between different code generators and languages, but we should at least be self-consistent within protobuf-ts.

protobuf-ts/packages/proto/google/protobuf/unittest.proto

Lines 568 to 570 in 3a7ce47

	// We list fields out of order, to ensure that we're using field number and not
	// field index to determine serialization order.
	message TestFieldOrderings {