Added coercive calls and tests

coercive constructor calls have the form: int32x4(x) where x is already an int32x4. More explanation is available in one of the SIMD email threads. Current behavior if the input actually isn't an int32x4 is to throw; logical extensions could be: 
1) if the input x is an object, put x.x in the first lane, x.y in the second lane, etc.
2) put ToInt32(x) in all lanes (which would be equivalent to splat)