simplify API to avoid (and raise on) user errors

This fixes our API to match the documentation in making
Msgthr#order! optional.  Furthermore, the block previously
passed to Msgthr#order! may now be passed to Msgthr#thread!
instead.

We accomplish this by tracking internal state explicitly, so a
Msgthr::StateError exception will be raised when methods are
called in an unsupported order.  This internal state is reset
with Msgthr#clear.

For users who truly do not care about ordering, Msgthr#walk_thread
may be called immediately after the last call to Msgthr#add.

Thanks to Dimid Duchovny for the feedback which led to this:
https://80x24.org/msgthr-public/CANKvuDc2mkxLuh+3+WXWfMXzxK2bShNesrD5xLocGOD1RybbwQ@mail.gmail.com/

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