The only such library I know currently is Akka DataFlow: http://doc.akka.io/docs/akka/snapshot/scala/dataflow.html
Looking at the project definition (https://github.com/akka/akka/blob/master/project/AkkaBuild.scala), it seems that akka-dataflow does not have much dependencies, so it should be pretty lightweight.
UPDATE: Here's an example use, taken from akka-dataflow's documentation:
flow {
val f1 = flow { "Hello" }
val f2 = flow { "world!" }
f1() + " " + f2()
} onComplete println
Which clearly illustrate that you can write your code in a procedural way (even though under the hood this really is transformed into a map/flatMap chain).
As another smple alternative, have you considered just using for continuations? They are bit more verbose to use, but the general benefit is the same: writing your code procedurally instead of cluttering it with explicit cps-style. By example, the above example directly translates to the following pure scala code:
{for {
f1 <- future( "Hello" );
f2 <- future( "world!" )
} yield f1 + " " + f2
} onComplete println
Which is not that different. And you won't find a more lightweight library than no library (though I admit that I still prefer akka-dataflow in terms of conciseness, but only by a slight margin).