option: Title-case `Some` and `None` in docs and fail messages

For accuracy, say 'get_ref None' instead of 'get_ref none', and so on.
option: Add .chain_mut_ref()

.chain_mut_ref() is the mutable alternative to .chain_ref().

A use case example for this method is extraction of an optional value
from an Option<Container> value.
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.