Add basic support for explicit type annotations. This is done by inserting a...

Add basic support for explicit type annotations. This is done by inserting a no-op function call. Note that this is meant as fallback, and we prefer the following alternatives (in their order) for inferring the type:
 1. Automatic from context, e.g. the type of a list based on the elements added to it (WIP)
 2. Type annotations (Python 3.6+ only)

PiperOrigin-RevId: 188120527