retrieveManaged/lib_managed no longer needed

now that our auxiliary scripts like benches.scala and profiles.scala
run through sbt instead of doing their own classpath-building, we
don't need to copy our managed jars into lib_managed anymore, we can
just go with the default sbt 0.10+ behavior of leaving them in
~/.ivy2/cache
fix up incorrectly rejiggered link-length prim

happened to notice this while doing something else. nbd, but the
bytecode generator can do a better job when this is right
bytecode generator: beef up MethodSelectorTests a little
give all report/report_* methods explicit return types

the main impetus for this was that in Scala 2.11, if you override
an AnyRef-returning method the compiler will no longer infer a more
specific return type if it can. the more specific return types are
valuable because the bytecode generator can use them.

committing this even for use with 2.10 because it seems like a
code improvement, actually; better not to rely on type inference
for something so crucial as the return types of these methods.

the benchmark dumps change a little, for the better, because some
int-to-double conversions are now happening before bytecode generation
Merge branch 'headless' into scala-2.11