Make Call.loop @tailrec optimized.

[TomasMikula]

Ensure Call.loop is @tailrec optimized. Elegance of the code suffered a bit, but I guess it's worth it?

The fact that @tailrec could not be put on loop as it was actually seems like a Scala bug, because the second call to loop is not a recursive call.

[codecov-io]

Current coverage is 88.98%

Merging #923 into master will increase coverage by +0.04% as of 819aa1f

@@            master    #923   diff @@
======================================
  Files          179     179       
  Stmts         2125    2125       
  Branches        42      42       
  Methods          0       0       
======================================
+ Hit           1890    1891     +1
  Partial          0       0       
+ Missed         235     234     -1

Review entire Coverage Diff as of 819aa1f

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[ceedubs]

👍. Ensuring @tailrec optimization seems like a good enough reason to make this minor sacrifice in code cleanliness.

[stew]

👍