Support integer64 in cor(), lm()?

```r
library(bit64)
x <- as.integer64(c(2, 3, 6, 6))
y <- as.integer64(c(3, 5, 5, 6))
cor(x, y)
# [1] NaN

lm(y ~ x)
# Call:
# lm(formula = y ~ x)

# Coefficients:
# (Intercept)            x  
#         NaN          NaN  
```