Inline Finallyalizer::drop, allowing LLVM to optimize `finally`.
* fixes the vec::from_elem regression caused by #8780
* added 5 benchmarks for allocating a 1KB ~[u8] and zeroing it
auto merge of #10918 : eddyb/rust/inline-finally-dtor, r=thestinger

* fixes the vec::from_elem regression caused by #8780
* added 5 benchmarks for allocating a 1KB `~[u8]` and zeroing it
* closes #7136