Implement SIMD.js API update

It includes:
 * Replaced SIMD.XXX.toYYY() conversion operators with SIMD.YYY.fromXXX() operators
 * Added corresponding bitwise conversion operators named SIMD.YYY.fromXXXBits()
 * Introduced SIMD.XXX.select() operators for float32x4 and int32x4
 * Introduced 1-argument constructor for float32x4, float64x2 and int32x4