Add Deref implementations for wrappers.

[andy-thomason]

This PR adds derefs for wrapper types.

Vectors deref to slices of their value (Rint /Rfloat/Rbool/Rstr/Robj).

Some Vectors deref to mutable slices (Rint/Rfloat/Rbool) but not Rstr and Robj which need set_elt to work.

ExternalPtr derefs to &T so we can use it like Box.

Rstr derefs to &str

We could deref Robj to &Rany - answers on a postcard.

[andy-thomason]

This will enable us to do:

fn fred(x: Integers) -> Rint {
   x[2]
}

fn fred(x: List) -> Robj {
   x[2].clone()
}

[yutannihilation]

Looks very useful!