[Bugfix] fix denodeify of ext/promise

proto to was not set correctly
[Bugfix] reduce boot time costs

nothing ground breaking, just some refactoring to
ensure we don’t end up with a super slow start time.

old: ember version  0.81s user 0.08s system 97% cpu 0.959 total
new: ember version  0.31s user 0.04s system 98% cpu 0.350 total
Merge pull request #1191 from stefanpenner/boot-speedup

Boot speedup