Add benchmarks for smallvec when it's on the stack

This makes all the benchmarks generic so the addition of the `_small` variants didn't totally blow up the line count, which as an added bonus also means we don't get duplication between the vec and smallvec benchmarks.

Results:

```
 name                           vec.bench ns/iter          smallvec.bench ns/iter          diff ns/iter   diff %  speedup
 bench_extend                   70                         102                                       32   45.71%   x 0.69
 bench_extend_from_slice        65                         52                                       -13  -20.00%   x 1.25
 bench_extend_from_slice_small  29                         24                                        -5  -17.24%   x 1.21
 bench_extend_small             28                         25                                        -3  -10.71%   x 1.12
 bench_from_slice               34                         97                                        63  185.29%   x 0.35
 bench_from_slice_small         31                         36                                         5   16.13%   x 0.86
 bench_insert                   1,235                      1,224                                    -11   -0.89%   x 1.01
 bench_insert_small             114                        117                                        3    2.63%   x 0.97
 bench_macro_from_elem          45                         55                                        10   22.22%   x 0.82
 bench_macro_from_elem_small    33                         17                                       -16  -48.48%   x 1.94
 bench_macro_from_list          33                         47                                        14   42.42%   x 0.70
 bench_push                     379                        462                                       83   21.90%   x 0.82
 bench_push_small               53                         57                                         4    7.55%   x 0.93
 bench_pushpop                  252                        307                                       55   21.83%   x 0.82
```

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Changes in this release:

* Fix possible double-free in `insert_many` when passed an iterator that
  panics in `next` (servo#103)
* Add a new `union` feature (disabled by default) that reduces the size
  of the SmallVec struct (servo#94)
* Improve performance of `extend` and `from_elem` (servo#93)
* Improve performance of `drop` (servo#100)
* Update dev-dependency on `bincode` (servo#102)
* Update to build without `std` on current Rust nightly (servo#104)
* Additional benchmarks (servo#95, servo#97).

Changes in this release:

* Fix possible double-free in `insert_many` when passed an iterator that
  panics in `next` (servo#103)
* Add a new `union` feature (disabled by default) that reduces the size
  of the SmallVec struct (servo#94)
* Improve performance of `extend` and `from_elem` (servo#93)
* Improve performance of `drop` (servo#100)
* Update dev-dependency on `bincode` (servo#102)
* Update to build without `std` on current Rust nightly (servo#104)
* Additional benchmarks (servo#95, servo#97).

Version 0.6.3

Changes in this release:

* Fix possible double-free in `insert_many` when passed an iterator that panics in `next` (#103)
* Add a new `union` feature (disabled by default) that reduces the size of the SmallVec struct (#94)
* Improve performance of `extend` and `from_elem` (#93)
* Improve performance of `drop` (#100)
* Update to build without `std` feature on current Rust nightly (#104)
* Additional benchmarks (#95, #97)
* Update dev-dependency on `bincode` (#102)

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