[feature request] support for bit64::integer64

[dselivanov]

Hi Alex. Would it be possible to add serialization of integer64 from bit64 package?

rjson::toJSON(list(bit64::as.integer64(111)))
#"[5.48412866883784e-322]"

bit64 pkg uses R's double vectors to keep 64 bit integers (so essentially it is numeric vector with class = "integer64" attribute). Underlying memory layout is the same as C/C++ int64_t.

[alexcb]

if you can make it work without introducing bit64 as a hard dependency, feel free to submit a PR.