Add incremental mode (#1292)

Key changes:

- Total rewrite of the module processing logic in build.py
- Support serialization to and deserialization from JSON for most classes in nodes.py and types.py
- When -i/--incremental command line flag is used, use a JSON cache for symbol tables of error-free modules

More discussion in the Comments section of the PR (#1292).

Author: gvanrossum

.gitignore

@@ -9,6 +9,7 @@ docs/build/
 *.iml
 /out/
 .venv/
+.mypy_cache/
 
 # Packages
 *.egg