Dynamic class creation by "guessing" types

We already have support for dynamic class creation, but it assumes that the types for all fields are known in advance: #1179

However, it would be nice to be able to dynamically create classes from data alone. We would have to "guess" the types by looking at the data itself (e.g. in the dict).

This would be useful, for example, for loading data from csv without having a schema beforehand. Reference: #1144