Display generator expressions

I think this needs more thought, but this is a first attempt. Generator expressions aren't currently displayed, because you don't know the value when they are assigned. However, the report builder could build up the display as the generator is consumed by later code.

Here's the current display for a small example:

s = 'xYz' | s = 'xYz'
a1 = (c.upper() for c in s) |
a2 = [c.upper() for c in s] | a2 = ['X', 'Y', 'Z']
a3 = list(a1) | a3 = ['X', 'Y', 'Z']

A display like `a1 = ('X', 'Y', 'Z')` might be confusing, because that looks like a tuple. This is the best I can think of:

s = 'xYz' | s = 'xYz'
a1 = (c.upper() for c in s) | a1 = gen('X', 'Y', 'Z')
a2 = [c.upper() for c in s] | a2 = ['X', 'Y', 'Z']
a3 = list(a1) | a3 = ['X', 'Y', 'Z']

The issue names no files or tests. Start by tracing the report builder's handling of generator expressions and compare the displayed examples; done means choosing and implementing an unambiguous representation that reflects consumption without presenting a generator as a tuple.

Written by the indexing model from the issue text.