Reference Dagger.EAGER_THUNK_STREAMS explicitly

[JamesWrigley]

A lot of tests failed when I was running them locally because that variable is defined in the Dagger module rather than Dagger.Sch. Though a lot still fail for me with this error:

  Got exception outside of a @test                                                                                                                                                                                                             
  On worker 2:                                                                                                                                                                                                                                 
  Tuple field type cannot be Union{}

I think we're running into JuliaLang/julia#52385 😬 I'll try to look into that in a few days.

(P.S.: I configured the PR to merge into #463 instead of master)

[JamesWrigley]

I think this should be fixed in 73c62dc, where I changed the code to use Base.promote_op(). I'm not an expert with that part of Julia though so I may be mistaken.

[jpsamaroo]

Thanks, this avoids the heartburn I feel when using Base._return_type 😄