Remove duplicated function resulting from merge.
Use existing API function rather than introducing a subtly different new one.

Since the scopes module passes requests upwards to outwards scope, we don't even
need lookup_name to do the same. [In other words, previously we were doing two
lookups, which, whilst not having any observably incorrect effects, was not
exactly efficient.]
Always promote get_cell's arguments.

I should probably have done this originally. Gives a 2% or so increase to
Richards.
Merge branch 'master' into pypy_bridge