[#2428] use external pointers to refer to objects backed by R_raster_tmp files

Personally, I've never had to use external pointers in my own code. RODBC uses them and it's cited as an example in Writing R Extensions 5.13, so I'll use it as an example as well.

> channel <- odbcConnectExcel("test.xls")
> rm(channel)
> invisible(gc())
Warning: closing unused RODBC handle 1
> channel <- odbcConnectExcel("test.xls")
> channel2 <- channel
> rm(channel)
> invisible(gc())
> rm(channel2)
> invisible(gc())
Warning: closing unused RODBC handle 2

I imagined the external pointer would just point to the filename. The read code would have to be modified to extract the string first, but then everything else should be the same. The external pointer gets copied around, but it's not finalized until the last reference is destroyed, as with channel and channel2. The finalizer code would delete the temporary file, and would get called on exit. If it's coded similarly to how RODBC works, it should be safe as long as nobody hunts down the temporary file and creates a regular file reference to it, but nobody should be doing that in the first place. I don't know how much magic is coded in RODBC to get it to work this way, but it should be somewhat easier with example code.

Anyway, just a thought. I've been just writing my code to avoid creating these temporary objects, and I think it's forcing me to write more efficient code anyway.