Replace `is_equal()` with `identical()`

[reikookamoto]

Consider replacing custom function with identical() from base R.

> identical(1, 2)
[1] FALSE
> identical(1, 1)
[1] TRUE
> identical(1, NA)
[1] FALSE
> identical(NA, NA)
[1] TRUE

This is identical to the behaviour in the example section of the documentation.

[reikookamoto]

@yulric Do you have a preference?

[yulric]

Sounds good i.e. I prefer the identical function! The more base R functions we use the better.