[tqchen]

downcast_ref and Array indexing now borrow DLDataType, DLDevice and BigInt in place, as they do i64 and f64. DLDataType and DLDevice are stored in the cell's union as themselves, and a BigInt is a whole cell, like Any.

let dtype: &DLDataType = value.downcast_ref::<DLDataType>().unwrap();
let n: &BigInt = value.downcast_ref::<BigInt>().unwrap();

These two commits were pushed to #935 after it was reviewed and did not land with it.