Minor fixes and simplifications

This includes the "use strict" removal. I looked at doing the refactoring suggested in #73, but it adds significant clutter. As a ligher-weight alternative, I added "use strict" to the benchmark script, which should make it easy to ensure that the code remains strict-clean.
I use `Int8Array` before since it supports to load from any `byteOffset`. By changing it to `Float32Array`, the `byteOffset` must be a multiple of 4. This can be tested as:
```
var a = new Int8Array(100);
SIMD.float32x4.load(a.buffer, 1);
```
It will raise `RangeError: start offset of Float32Array should be a multiple of 4` exception.
@sunfishcode @johnmccutchan