auto merge of #7931 : blake2-ppc/rust/chain-mut-ref, r=pcwalton

auto merge of #7931 : blake2-ppc/rust/chain-mut-ref, r=pcwalton

First, clean up the uses of "None" and "Some" to always use consistent title case matching the variant names.

Add .chain_mut_ref() which is a missing method. A use case example for this method is extraction of an optional value from an Option\<Container\> value.