Supply missing argument to choose_heap_size()

Fixes issue #538
Merge pull request #539 from jaredhoberock/issue-538

Supply missing argument to choose_heap_size()
Eliminate use of uninitialized<Closure> in launch_closure to avoid significant performance regression
Merge pull request #540 from jaredhoberock/launch_closure

Eliminate use of uninitialized<Closure> in launch_closure to avoid signi...