ops64 dispatch with both arguments having classes

[hcirellu]

The default S3 dispatch on the Ops group for two arguments having class attributes results in a dispatch of the default method with a warning. Since R 4.3.0 one can use chooseOpsMethod to handle that situation by signaling which method of the two classes should be dispatched to.

Since difftime * integer64 or integer64 * difftime should return difftime I use the helper target_class_for_Ops() to determine whether to return integer64 or not. Everytime when a integer64 argument is paired with a logical, (plain) integer, integer64, or (plain) double argument integer64 is returned. Otherwise the integer64 argument is coerced to double and the method of the class of the other argument (or default) is called.

I moved all the methods of the Ops group into a new file ops64.R with a corresponding man page.

I figured out, that an explicit handling of xor is not necessary, since the default functions (either base or bit) work and have a consistent result with base R.

I added tests for a consistent behaviour of R for the data types logical, integer, double, complex, Date, POSIXct, POSIXlt, and difftime for all Ops operators including xor.

(During testing I found #248.)

Closes #179