Add another escape for things evaluated before the function definition

You often see

@rsubset df :a in Set([1, 2])

this is super slow because you have to make Set([1, 2]) evaluated every time.

The following works

julia> transform(df, let 
           y = 1
           :a => (t -> t .+ y) => :c
       end)
1×3 DataFrame
 Row │ a      b      c     
     │ Int64  Int64  Int64 
─────┼─────────────────────
   1 │     1      2      2

julia> y
ERROR: UndefVarError: y not defined

We could add some sort of escape character, like ^ or $ for things that get evaluated before the function is defined.

Start by reproducing the @rsubset example and comparing it with the working transform(let ...) example in the issue. Before implementation, determine the intended escape syntax and its evaluation semantics with maintainers; done should include a documented syntax and tests showing values such as Set([1, 2]) are evaluated before the function definition.