Make StreamingT toString more like Streaming toString

[ceedubs]

Before this, StreamingT was always returning "StreamingT(...)" for its
toString. It's now similar to Streaming in that it will return
"StreamingT()" in the case of Empty and will show the head element in
the case of Cons.

I also added a pretty weak test for its toString (which actually doesn't
capture my changes at all). It may be good to add further tests in the
future, but I've got to get to work, and I think this is at least an
improvement to the current status.

[codecov-io]

Current coverage is 85.58%

Merging #722 into master will increase coverage by +0.06% as of 986fdc6

@@            master    #722   diff @@
======================================
  Files          162     162       
  Stmts         2245    2247     +2
  Branches        74      74       
  Methods          0       0       
======================================
+ Hit           1920    1923     +3
  Partial          0       0       
+ Missed         325     324     -1

Review entire Coverage Diff as of 986fdc6

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[adelbertc]

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