Decide how to deal with int64

[Bisaloo]

Currently, int64 (as well as uint32 and uint64) are converted to NA_integer_ if they overflow during the conversion to R native integer type, int32.

To support larger values than what fits in int32, we have several options:

• convert large integers to double. This is what reticulate does.
• convert larger integers to bit64.
• let the user decide if they want NA, double or bit64. This is what rhdf5 does.