ringbuf: Implement .size_hint() for iterators
dlist: Simplify by using Option::{map, map_mut}

These methods were fixed or just added so they can now be used.
dlist: Use inline on very small functions and iterator functions
auto merge of #7790 : blake2-ppc/rust/dlist-ringbuf-small-changes, r=thestinger

Implement size_hint for the ringbuf iterators.

Do small cleanups in dlist, use Option's .map and .map_mut properly, and put inline on all the small methods.