Add collection element and map k/v type gen for random class instance

[serpro69]

Fix #242

TODO:

• update docs
• in case of maps, do we want to be able to predefine generators for keys as well? (I can see this not being very useful and even harmful sometimes, e.g. when k/v are the same type like String)
  • try if we can make it "parameterized" and allow the user to specify whether they want to generate keys, values or both in a Map