Adapt to utils:::print.bitstring

[MichaelChirico]

Closes #295

cc @deepayan

Beyond just passing my silly test, here's the "actual experience" of using bitstring on ancient R:

library(bit64)

# the correct method is registered by .onLoad when it's missing from {utils}
getS3method("print", "bitstring")
# function (x, ...) 
# {
#     reset_class = minusclass(class(x), "bitstring")
#     attributes(x) = NULL
#     oldClass(x) = reset_class
#     NextMethod(x)
# }
# <bytecode: 0x5c590d7083e8>
# <environment: 0x5c590d7021d8>

# the new attributes are omitted
print(as.bitstring(lim.integer64()))
# [1] "1000000000000000000000000000000000000000000000000000000000000001"
# [2] "0111111111111111111111111111111111111111111111111111111111111111"

[MichaelChirico]

devel issue is an ongoing {rlang} outage; ignoring.