Use Vec::with_capacity() in multi::count.
We know how many entries are expected so we can avoid many
reallocations.
I'm parsing btree nodes from disk and typically have to parse arrays
of ~150-250 u64s.  This changes reduces the run time from 41 seconds to
36 seconds.
@@ -633,7 +633,7 @@ where
{
move |i: I| {
let mut input = i.clone();
- let mut res = crate::lib::std::vec::Vec::new();
+ let mut res = crate::lib::std::vec::Vec::with_capacity(count);
for _ in 0..count {
let input_ = input.clone();
