Add a standard subsetting promoter

Promote a standard accessor strings, probably limited to `$`, `[`, `[[`, `@`, to a lens e.g

```r
promote_l( ~ .[[1]]$col[[2]]@slt[1:5] ) 
```

equal to

```r
index(1) %.% 
  index("col") %.% 
  index(2) %.% 
  slot_l("slt") %.% 
  indices(1:5)
```