layer try/except blocks

We should layer statements in each block so that we can accumulate state without breaking scope and still catch exceptions.  This will unify `var x = foo()` statements with the rest of the tests.
```nim
var x = foo()
test2()
```
...turns into...
```nim
try:
  var x = foo()
  try:
     test2()
  except: ...
  finally: ...
except: ...
finally: ...
```