Fix infinite loop in NSCFDictionary/NSCFSet fast enumeration

[rupertdaniel]

Summary

-[NSCFDictionary countByEnumeratingWithState:objects:count:] and the NSCFSet equivalent created a new enumerator on every call:

NSEnumerator *enuM = [self keyEnumerator];   // fresh enumerator each call
return [enuM countByEnumeratingWithState: state objects: stackbuf count: len];

The for/in fast-enumeration protocol invokes this method repeatedly until it returns 0, relying on NSFastEnumerationState to carry progress between calls. Because a brand-new enumerator restarts from the beginning every time, the method never returns 0 — so enumerating any toll-free-bridged CFDictionary/CFSet (e.g. one created with CFDictionaryCreateMutable) loops forever over the first batch of elements.

Fix

Iterate the backing GSHashTable directly, keeping the resume bucket index in state->extra[0]. A new private helper GSHashTableGetKeysFromCursor() reads live buckets in place, so there is no per-call allocation and no snapshot to free/leak. This mirrors the cursor-in-extra[] technique gnustep-base's own GSDictionary uses for fast enumeration.

• Source/GSHashTable.{c,h} — add GSHashTableGetKeysFromCursor().
• Source/NSCFDictionary.m, Source/NSCFSet.m — implement countByEnumeratingWithState:objects:count: on top of it.

NSFastEnumerationState stays confined to the .m bridge files; the C core gains only a plain CFIndex/const void ** iterator, so no Foundation dependency is introduced into GSHashTable.

Testing

Built with the MSVC toolchain and verified that for (id key in dict) over a CFDictionaryCreateMutable-created dictionary now enumerates each element once and terminates, where it previously looped indefinitely.

[triplef]

@HendrikHuebner do you mind reviewing this one as well? We ran into this in our app.

[HendrikHuebner]

LGTM. Do we have any tests for this?

[HendrikHuebner]

Suggested change

	* bucket *cursor, advances *cursor past them, and returns the number written
	* (0 once the table is exhausted). Reads live buckets in place and allocates
	* nothing, so it is suitable as a fast-enumeration primitive.
	* bucket *cursor, advances *cursor past them, and returns the number of written keys
	* (0 once the table is exhausted).