Support Iterator-like style for shrink implementation

[vlsi]

Currently shrink causes lots of objects to be created because doShrink should return List.

It guess it would be better if shrinks could be added iteratively, so the verifier could pick objects and verify them. If property still fails, it could try shrinking the object that fails.

I guess it would produce more targeted results while current API does not make it clear how many shrink results should be produced.

[pholser]

@visi Sounds reasonable -- I'll investigate.

[vlsi]

This duplicates #220