According to https://docs.python.org/3/library/ast.html#ast-helpers, we expect type_ignores to be always be empty, so this adds a debug assert.
Test plan: I confirmed that the assertion holdes for the file below and for all the black tests which include a number of `type: ignore` comments.
```python
# type: ignore
if 1:
print("1") # type: ignore
# elsebranch
# type: ignore
else: # type: ignore
print("2") # type: ignore
while 1:
print()
# type: ignore
```
* Implement module formatting using JoinNodesBuilder
This uses JoinNodesBuilder to implement module formatting for #4800
See the snapshots for the changed behaviour. See one PR up for a CLI that i used to verify the trailing new line behaviour
* Add Format for Stmt
* Implement basic module formatting
This implements formatting each statement in a module with a hard line break in between, so that we can start formatting statements.
Basic testing is done by the snapshots
konstin