int64_t batch_size = 0;

if (BSON_ITER_HOLDS_INT32 (&iter) || BSON_ITER_HOLDS_INT64 (&iter) || BSON_ITER_HOLDS_DOUBLE (&iter)) {
    value = bson_iter_as_int64 (&iter);
}

This is perfectly valid code, but is annoying to write bunch of times.

We should add two new macros:

• BSON_ITER_HOLDS_NUMBER()
  • Checks int32, int64 & double
• BSON_ITER_HOLDS_INT()
  • Checks for int32 & int64

We should probably also go and fix code to use these.