[Rd] Object.size() should not visit every element for alt-rep strings, or there should be an altstring_objectsize_method

[Travers Ching]

Below is a toy alt-rep string example, that generates N random strings:

https://gist.github.com/traversc/a48a504eb062554f2d6ff8043ca16f9c

example:
`x <- altrandomStrings(1e8)`
`head(x)`
[1] "2PN0bdwPY7CA8M06zVKEkhHgZVgtV1" "5PN2qmWqBlQ9wQj99nsQzldVI5ZuGX" ...
`object.size(1e8)`

Object.size will call the `set_altstring_Elt_method` for every single
element, materializing (slowly) every element of the vector.  This is
a problem mostly in R-studio since object.size is called
automatically, defeating the purpose of alt-rep entirely.