[m-rev.] for possible review: use explicit streams in the MLDS backend

[Zoltan Somogyi]

Does anyone want to review this? It contains more of the same kinds
of changes as the previous diff, and it is pretty boring. It has bootchecked
in hlc.gc, csharp and java grades. In the last two grades, it had a few more
test failures than the last bootchecks I did in those grades, but I haven't
seen any evidence that they were caused by this change.

Zoltan.
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