Remove unused `P.guard`; replace with `Promise.guard` from `prfun`.

Change-Id: I9cd4ba9de0cb2fc1f95ff3d28982849c651107d2
Downgrade `readable-stream` dependency.

The docs for the `readable-stream` package say to use version "~1.0.0"
for Streams2 compatibility.  The 1.1.x is reserved for "streams3",
whatever that turns out to be.  At the moment, Streams2 is good enough
for us.

Change-Id: I3ba167d3ad1cce3d736543c6da2b83085ecd71e3