Rename reserve to reserve_exact and reserve_at_least to reserve

Changes in std::{str,vec,hashmap} and extra::{priority_queue,ringbuf}.
Fixes #11949
auto merge of #11951 : dmanescu/rust/reserve-rename, r=huonw

Changes in std::{str,vec,hashmap} and extra::{priority_queue,ringbuf}.
Fixes #11949