Export objc_enumerationMutation() function

[triplef]

Fixes linker errors on Windows when using for-in loops.

[davidchisnall]

I don't believe that weak symbols are not preemptable (or, in fact, supported) on Windows, so for Windows we probably need to:

• Make this invoke a callback.
• Add a function to register the callback.

[triplef]

Good to know. This is in fact how it’s done on Apple platforms via objc_setEnumerationMutationHandler().