Auto merge of #94 - arthurprs:union, r=mbrubeck

Use a union to reduce the size of SmallVec [v2]

Building on top of #92 by @Amanieu

I introduced `triple()` and `triple_mut()` which removed almost all of the runtime overhead. Performance is very comparable.

```
 name                     master:: ns/iter  union:: ns/iter  diff ns/iter   diff %  speedup
 bench_extend             45                47                          2    4.44%   x 0.96
 bench_extend_from_slice  45                43                         -2   -4.44%   x 1.05
 bench_from_slice         45                44                         -1   -2.22%   x 1.02
 bench_insert             615               622                         7    1.14%   x 0.99
 bench_insert_from_slice  101               99                         -2   -1.98%   x 1.02
 bench_insert_many        309               266                       -43  -13.92%   x 1.16
 bench_macro_from_elem    41                37                         -4   -9.76%   x 1.11
 bench_macro_from_list    40                42                          2    5.00%   x 0.95
 bench_push               381               370                       -11   -2.89%   x 1.03
 bench_pushpop            404               420                        16    3.96%   x 0.96
 bench_remove             458               436                       -22   -4.80%   x 1.05
```

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Author: bors-servo

.travis.yml

@@ -5,10 +5,9 @@ rust:
   - stable
 script: |
   cargo build --verbose &&
-  cargo build --all-features --verbose &&
   cargo test --verbose &&
-  cargo test --all-features --verbose &&
   ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --verbose --no-default-features) &&
+  ([ $TRAVIS_RUST_VERSION != nightly ] || cargo test --verbose --features union) &&
   ([ $TRAVIS_RUST_VERSION != nightly ] || cargo bench --verbose bench)
 notifications:
   webhooks: http://build.servo.org:54856/travis

Cargo.toml

@@ -12,13 +12,15 @@ documentation = "http://doc.servo.org/smallvec/"
 
 [features]
 std = []
+union = []
 default = ["std"]
 
 [lib]
 name = "smallvec"
 path = "lib.rs"
 
 [dependencies]
+unreachable = "1.0.0"
 serde = { version = "1", optional = true }
 
 [dev_dependencies]

benches/bench.rs

@@ -158,15 +158,15 @@ fn gen_insert<V: Vector<u64>>(n: u64, b: &mut Bencher) {
 
 fn gen_remove<V: Vector<u64>>(n: usize, b: &mut Bencher) {
     #[inline(never)]
-    fn remove_noinline<V: Vector<u64>>(vec: &mut V, p: usize) {
-        vec.remove(p);
+    fn remove_noinline<V: Vector<u64>>(vec: &mut V, p: usize) -> u64 {
+        vec.remove(p)
     }
 
     b.iter(|| {
         let mut vec = V::from_elem(0, n as _);
 
         for x in (0..n - 1).rev() {
-            remove_noinline(&mut vec, x)
+            remove_noinline(&mut vec, x);
         }
     });
 }