Int32 constructor should coerce its argument

The Int32 constructor should coerce its argument immediately to int32, so that e.g. Int32(-(2**40)).valueOf() === 0 instead of Int32(-(2**40)).valueOf() === -(2**40). This makes it consistent with e.g. Long, and makes Int32 match what is being serialized when encoding as BSON.

AC

• Make sure the behavior matches what is described above
• Test numbers outside the range of Int32, positive and negative