Method lazyvec() duplicates a range

[MarcusKlik]

By using R based ALTREP methods.

Phase 1 of lazyvec():

The default implementation of a newly generated lazyvec mimics a range n:m with all methods implemented in R.

[MarcusKlik]

The code for sample ALTREP implementations could be provided on the project home page generated from articles in the package (using pkgdown). That way, users can access that code from the package help or on the website.