Better support for error/hint location

[ilevkivskyi]

In view of planned editor integration support I think there are two important things to do in addition to fixing relevant high-priority issues like #3871 that require updating typed_ast:

• Make a thorough walk through the code and fix all places where column is not set correctly, there are still quite a few such places I think, see also Check coulmn numbers in existing tests #4276.
• Design an API to quickly find a node object by file name, line number, and column. This will be needed at later stages when we will need to return type of a node (or some other info like accessible attributes) by request from the editor.

[JukkaL]

Column numbers are now set pretty reliably already (#7053). The second goal is no longer a priority (but it's possible that we'll revisit the idea at some point in the future).