symtable as a generic visitor

[s-cork]

The internals of each visitor are the same.

I skipped test 483 because the variables print and ZeroDivision switched places.
This was because the autogenerated ast visitor visits the orelse before the handlers (that's also what pypy's does).
whereas the cpython visitor visits the handlers before the orelse.

There were a few choices there.

1. skip the test (t483.py)
2. change the base generic visitor for visit_Try.
3. change the SymbolTable visitor for visit_Try.

I went with 1. But 3 would also be good.

Bench marks are very slightly quicker for this branch vs master. But it's nothing to write home about.

[albertjan]

Yeah I would've gone for 3 😄 looks cool though. Might make it harder to port newer version of cpython to this though 🤷

[s-cork]

Possibly. But thinking about the update process, if we need to update I guess we'd look at the symtable diff in cpython, work out where the changes were made then map the changes to our implementation.

In both implementations there's still a one to one correspondence for mapping the changes. If say the case for functionDef changed then instead of going to the switch case we just go to the visit_functionDef method.

Would be worthy of a top level comment I think explaining that to our future selves.