integer64 support

Originally posted in [mapdeck](https://github.com/SymbolixAU/mapdeck/issues/342)

```
library(colourvalues)
library(bit64)

x <- bit64::as.integer64(1:5)
colourvalues::colour_values(x)

# Error in rcpp_colour_values_hex(x = x, palette = palette, alpha = alpha,  : 
#  Error in function boost::math::trunc<double>(double): Value inf can not be represented in the target integer type.
```