Add C420 rule

[oliver-sanders]

For discussion...

Add a rule to recommend using generator expressions rather than list comprehensions when they are only used for iteration by a for loop e.g:

- for x in [func(y) for y in iterable]:
+ for x in (func(y) for y in iterable):
    ...

This is an inefficient pattern which seems to crop up every now and again.

The logic for this is similar to C419:

• List comprehensions prevent short-circuiting e.g:

  for x in [func(y) for y in iterable]:
      if x == 42:
         break  # we didn't need to call `func(y)` for items past this point
      do_something(x)

• List comprehensions create an intermediate list object, e.g. if iterable is 1'000 items long, then a 1'000 item list will be created in order for the for loop to iterate over which is inefficient.