Allow continue and break while iterating over a map

[tchajed]

In for...range loops over a map, we should support break similar to how ordinary for loops work. The concrete use case is code that finds an arbitrary element in a map for implementing an allocator.

This should reuse the existing code for chaining on an extra ; Continue when necessary, so we don't have to end every iteration with continue in Goose. This will also inherit improvements to that code to handle more control flow patterns.