Add a standard subsetting promoter

[cfhammill]

Promote a standard accessor strings, probably limited to $, [, [[, @, to a lens e.g

promote_l( ~ .[[1]]$col[[2]]@slt[1:5] ) 

equal to

index(1) %.% 
  index("col") %.% 
  index(2) %.% 
  slot_l("slt") %.% 
  indices(1:5)