debugger is very slow with large local variable strings

Consider this script:

``` py
def main():
d=['x',]*100000000
a='x'*10000000
b=1+1
print(b)

main()
```

set a breakpoint on `a=` and `b=`, then run to `a=`, then run to `b=`.

I'm fairly certain that this is caused by the Variables area spending a lot of time constructing a string.
For large lists as in `d=`, the Variables area is smart enough to show a slice and allow the user to ask for more. The same sort of intelligence would be incredibly valuable for strings.

It's really painful since the program I'm debugging passes around MBs or GBs of strings including returning them. It's perfectly fast w/o a debugger since they're just a reference.

FWIW, even `d` is a little slow with this size, one order of magnitude smaller and it isn't even noticeable, so there's room for improvement in arrays too.