FR: Add support for int64 vectors

[eddelbuettel]

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We have helper functions for int64_t vectors in a few places outside of Rcpp. It would probably be nice to bring some of that into Rcpp itself. Ideally without a hard dependency on bit64::as.integer64() so maybe this needs to go into a header-only add-on package bringing that dependency in.

The basic trick of the int64 ops though is to use double as a payload for int64_t data and memcpy to transfer which we may be able to support directly.