Make MsgStream.writeData() public

[snej]

Fixes #69

I assume this doesn’t need a unit test, because writeData is already called by other write methods so it’s already being tested.

[snej]

I think something’s wrong with your CI, since the only change in this PR is the addition of a * to make writeData public. I looked at some of the error messages but they don’t make sense to me and are unrelated to my change.

[jangko]

closing in favor of #71