Audit and fix method chain-ability in all POJOs

[toddbaert]

We have a few classes that are basically just POJOs (MutableContext, MutableStructure, etc) that have inconsistently chainable methods. All mutating methods should be chainable:

myPojo
  .addThing(thing)
  .increment()
  .addThing(anotherThing);

Audit all relevant classes to ensure we're consistent with this. Add or update tests to use any newly corrected chainable methods.

cc @Kavindu-Dodan if you have anything to add.