Document std::ops.

Added an overview with a 'real' example, as well as toy implementations of all of
the traits.

Closes #9356.
Update std::ops docs as per feedback

from @chris-morgan
auto merge of #9522 : steveklabnik/rust/doc_std_opts, r=brson

Added an overview with a 'real' example, as well as toy implementations of all of
the traits.

Closes #9356.