Add SIMD types load and store opertaions

They are useful to load/store vec1, vec2, vec3 or vec4 into/from
SIMD types from/into ArrayBuffer with byte offset.

The new APIs include:
- SIMD.float32x4.load
- SIMD.float32x4.loadX
- SIMD.float32x4.loadXY
- SIMD.float32x4.loadXYZ
- SIMD.float32x4.store
- SIMD.float32x4.storeXY
- SIMD.float32x4.storeXYZ
- SIMD.float64x2.load
- SIMD.float64x2.loadX
- SIMD.float64x2.store
- SIMD.float64x2.storeX
- SIMD.int32x4.load
- SIMD.int32x4.loadX
- SIMD.int32x4.loadXY
- SIMD.int32x4.loadXYZ
- SIMD.int32x4.store
- SIMD.int32x4.storeX
- SIMD.int32x4.storeXY
- SIMD.int32x4.storeXYZ
Merge pull request #65 from huningxin/load_store

Add SIMD types load and store opertaions