Generalize Tee to any channel

[destel]

Tee now accepts any channel, not only streams of Try. It never looked at the items it forwarded, so the Try constraint was incidental, and plain channels can now be duplicated the same way streams are.

Existing inferred call sites like Tee(stream) are backwards compatible and keep working.
Explicit instantiations such as rill.Tee[User](stream) must become rill.Tee[rill.Try[User]](stream).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.87%. Comparing base (049d498) to head (a5ee21f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files          17       17           
  Lines         806      806           
=======================================
  Hits          805      805           
  Misses          1        1           

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