add set operations

[hcirellu]

This PR adds support for set operations, i.e. union, intersect, setdiff, setequal and is.element, for integer64. Since in base these are functions, I used a regular overload and not an s3 method dispatch.

main

union(as.integer64(1L), 1L)
# [1] 4.940656e-324  1.000000e+00

this PR

union(as.integer64(1L), 1L)
# integer64
# [1] 1

Closes #182

(styling: I removed 6 white spaces.)