Move float32x4 and int32x4 constructors into SIMD module.

It is according to
https://github.com/johnmccutchan/ecmascript_simd/blob/b5dce3a7039a6d38cb7922f526cdb49a69f01dc5/src/ecmascript_simd.js
Merge pull request #34 from huningxin/update_simd

Move float32x4 and int32x4 constructors into SIMD module.