Support SortedMap properties

[alicederyn]

The issue that has blocked this is setting the comparator; TreeMap's comparator is immutable, but we want to return an unmodifiable view from getMySortedMapProperty(). Options:

1. Provide a custom SortedMap implementation in freebuilder.jar with a mutable comparator.
   Disadvantage: we don't currently expect users to ship freebuilder.jar.
2. Write out a SortedMap proxy inner class.
   Disadvantage: lots of code for a rarely-used feature.
3. Allow the comparator to be set only once, delaying TreeMap construction until then. The setter should probably be protected.
   Disadvantages:
   • users that want a mutable comparator will have to write one from scratch;
   • mergeFrom won't copy the comparator; and
   • no obvious path to migrate to solutions 1/2 in future, as users may rely on the behaviour of mergeFrom.

[cookingfox]

It would be great to have this.

To me option 2 sounds the most reasonable. Can you elaborate on how much code "lots" actually is? :)

Thanks!

[alicederyn]

SortedMap defines 20 methods, including some returning views that require more inner classes to be defined, adding dozens more methods.

[alicederyn]

An update on this issue: We now have SortedSet support, using option 3 above (comparator must be set before any other operation is called). Unfortunately it still requires a SortedSet proxy inner class to implement the mutate method; SortedMap will too.