Feature for preserving run-time gathered typing info (such as from tests)

[getify]

This is an interesting idea that fits well with TypL's vision, should explore having it incorporated.

https://twitter.com/mpjme/status/1127113416307630080

[getify]

Not only should we able to loop back in types discovered at test time, but pulling in types back in from previous runtime would be awesome as well.

One challenge: when we preserve types (say in a JSON file, etc), it needs to be hashed against the file contents, since any change to a file could invalidate all those bits of info.