Use NSStackBlock blocks for asynchronous methods

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Use NSStackBlock blocks for asynchronous methods in order to avoid the complicated lifecycle management.

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For reference, block specification and implementation:

• http://clang.llvm.org/docs/BlockLanguageSpec.html
• http://clang.llvm.org/docs/Block-ABI-Apple.html

Source code:

• https://llvm.org/svn/llvm-project/compiler-rt/trunk/BlocksRuntime/Block.h
• https://llvm.org/svn/llvm-project/compiler-rt/trunk/BlocksRuntime/Block_private.h
• https://llvm.org/svn/llvm-project/compiler-rt/trunk/BlocksRuntime/data.c
• https://llvm.org/svn/llvm-project/compiler-rt/trunk/BlocksRuntime/runtime.c