This reverts commit 79759fea, reversing
changes made to dc36f4df.

Unlike other Objective-C objects, there's no hard reason why the compiler wouldn't be able to initialize a block statically (as it does with NSString literals). And it certainly doesn't complain about it (like it does with other object initializers). But as I haven't been able to find confirmation of this, and we're seeing a weird crash occur near this code, let's play it safe.

Corrected compiler warning for effectively pure virtual ObjC methods.

Also clarify in GRXWriter.h that the writeable is released whenever the
writer finishes.

And move it to the RxLibrary.

With GRXWriter a subclass of all writers, we can map the requestsWriter directly
without a dumb wrapper, and return GRXImmediateWriter objects where GRXWriter is
expected.