is.nan.integer64() and is.infinite.integer64() don't return TRUE for NaN and Inf values respectively

[Rosie23]

is.nan.integer64 doesn't return TRUE for NaN values
is.infinite.integer64 doesn't return TRUE for Inf values

  is.nan.integer64(NaN) # FALSE
  as.integer64(NaN) # NA
  is.nan.integer64(as.integer64(NaN)) # FALSE
  
  is.infinite.integer64(Inf) # FALSE
  as.integer64(Inf) # NA Warning message: In as.integer64.double(Inf) : NAs produced by integer64 overflow
  is.infinite.integer64(as.integer64(Inf)) # FALSE

[MichaelChirico]

Inf and NaN are not integer64 values, they are doubles.

Also, don't call is.nan.integer64() or is.infinite.integer64() directly -- call is.nan() and is.infinite() and rely on S3 dispatch to do so.

is.infinite(NA_real_) is FALSE, so is.infinite(NA_integer64_) should also be FALSE.

We could add an error in is.nan.integer64() and is.infinite.integer64() if they are called with non-integer64 input, but I'd rather just unexport them to make doing so very difficult (require ::: access: #76.