factor::Decomposition: Inline a small number (4) of factors

This is ~2% faster, as we do not need to allocate on the heap for most numbers.

src/uu/factor/Cargo.toml

@@ -18,6 +18,7 @@ num-traits = "0.2" # used in src/numerics.rs, which is included by build.rs
 [dependencies]
 num-traits = "0.2"
 rand = { version="0.7", features=["small_rng"] }
+smallvec = "1.4"
 uucore = { version="0.0.4", package="uucore", git="https://github.com/uutils/uucore.git", branch="canary" }
 uucore_procs = { version="0.0.4", package="uucore_procs", git="https://github.com/uutils/uucore.git", branch="canary" }
 

src/uu/factor/src/factor.rs

@@ -7,6 +7,7 @@
 
 extern crate rand;
 
+use smallvec::SmallVec;
 use std::cell::RefCell;
 use std::fmt;
 
@@ -16,11 +17,16 @@ use crate::{miller_rabin, rho, table};
 type Exponent = u8;
 
 #[derive(Clone, Debug)]
-struct Decomposition(Vec<(u64, Exponent)>);
+struct Decomposition(SmallVec<[(u64, Exponent); NUM_FACTORS_INLINE]>);
+
+// The number of factors to inline directly into a `Decomposition` object.
+// As a consequence of the Erdős–Kac theorem, the average number of prime
+//  factors of integers < 10²⁵ ≃ 2⁸³ is 4, so we can use that value.
+const NUM_FACTORS_INLINE: usize = 4;
 
 impl Decomposition {
     fn one() -> Decomposition {
-        Decomposition(Vec::new())
+        Decomposition(SmallVec::new())
     }
 
     fn add(&mut self, factor: u64, exp: Exponent) {