auto merge of #15256 : erickt/rust/optimizations, r=alexcrichton

The bug #11084 causes `option::collect` and `result::collect` about twice as slower as it should because llvm is having some trouble optimizing away the scan closure. This gets rid of it so now those functions perform equivalent to a hand written version.

This also adds an impl of `Default` for `Rc` along the way.

diff --cc src/libcore/option.rs
Simple merge

diff --cc src/libcore/result.rs
Simple merge