[thin_metadata_pack/unpack] Use Vec::with_capacity() to avoid reallocs.

Gives a small speed boost to both pack and unpack.

src/pack/delta_list.rs

@@ -13,7 +13,7 @@ use Delta::*;
 pub fn to_delta(ns: &[u64]) -> Vec<Delta> {
     use std::cmp::Ordering::*;
     
-    let mut ds = Vec::new();
+    let mut ds = Vec::with_capacity(ns.len());
 
     if !ns.is_empty() {
         let mut base = ns[0];

src/pack/vm.rs

@@ -146,8 +146,8 @@ pub fn pack_u64s<W: Write>(w: &mut W, ns: &[u64]) -> io::Result<()> {
 }
 
 fn unshift_nrs(shift: usize, ns: &[u64]) -> (Vec<u64>, Vec<u64>) {
-    let mut values = Vec::new();
-    let mut shifts = Vec::new();
+    let mut values = Vec::with_capacity(ns.len());
+    let mut shifts = Vec::with_capacity(ns.len());
 
     let mask = (1 << shift) - 1;
     for n in ns {
@@ -207,7 +207,7 @@ fn unpack_with_width<R: Read>(r: &mut R, nibble: u8) -> io::Result<u64> {
 }
 
 fn unpack_u64s<R: Read>(r: &mut R, count: usize) -> io::Result<Vec<u64>> {
-    let mut v = Vec::new();
+    let mut v = Vec::with_capacity(count);
     for _ in 0..count {
         let n = r.read_u64::<LittleEndian>()?;
         v.push(n);