Support for functions called with [[ syntax

For example, profiling the following:

```r
x <- list(
  "one" = function() { Sys.sleep(3) },
  "two" = function() { Sys.sleep(3) }
)

x[["one"]]()
x[[2L]]()
```

currently yields lots of `<Unimplemented>`, despite the fact that it works with `Rprof()`.