ObjFW  Check-in [f5927f8a84]

This uses the runtime's autorelease pools and implements autorelease
pools in the ObjFW runtime. It therefore uses ObjFW's autorelease pools
when using the ObjFW runtime and Apple's autorelease pools when using
the Apple runtime.

These new pools should be ARC-compatible now and finally, it should be
possible to mix OFAutoreleasePools and NSAutoreleasePools again, even
@autoreleasepool is allowed in the mix now. This also means the old
bridge to NSAutoreleasePool should not be required anymore, as both use
the runtime's autorelease pools now.

As a bonus, it's significantly faster to use the ObjFW runtime with
ObjFW's autorelease pools than to use Apple's runtime with Apple's
autorelease pools, as a quick benchmark using OFXMLParser on large files
showed. (Note: This is not only due to the different autorelease pools,
but also due to the fact that even with the same autorelease pools it is
faster using the ObjFW runtime, as can be seen in versions prior to this
commit.)