More consistent API of dask.optimize functions that accept `keys` argument.
More consistent API of dask.optimize functions that accept `keys` argument.

Like for `get` functions, keys may be a single key or a possibly nested list
of keys that get flattened via `flatten`.  Internally, sometimes we deal with
sets of keys, so dask.optimize also accepts sets.

mikegraham/dask,vikhyat/dask,freeman-lab/dask,simudream/dask,ssanderson/dask,blaze/dask,pombredanne/dask,PhE/dask,wiso/dask,clarkfitzg/dask,jakirkham/dask,simudream/dask,gameduell/dask,ContinuumIO/dask,dask/dask,mrocklin/dask,mraspaud/dask,cpcloud/dask,pombredanne/dask,wiso/dask,mrocklin/dask,vikhyat/dask,clarkfitzg/dask,freeman-lab/dask,jayhetee/dask,PhE/dask,mraspaud/dask,jakirkham/dask,jcrist/dask,dask/dask,blaze/dask,jayhetee/dask,chrisbarber/dask,ContinuumIO/dask,ssanderson/dask,cowlicks/dask,jcrist/dask