expose `_get_body`

[thautwarm]

_get_body extracts the type encoding of the body of GG functions.

 julia> fun = mk_function(Main, :(function (x) x + 1 end))
function = (x;) -> begin
    begin
        (Main).:+(x, 1)
    end
end

julia>  from_type(GeneralizedGenerated._get_body(fun))

quote
    begin
        #= REPL[27]:1 =#
        #= REPL[27]:1 =#
        (Main).:+(x, 1)
    end
end

This is very useful to control scopes.

For instance, if we want to specify bind values to local variables in the generated body, we can firstly create a dummy function and specify arguments which will be bound to our values.

@gg function f(M::Module, a, b)

   c = 1
   fun = mk_function(M, :(function (a, b, c) 
       ...
   end))

   body =  from_type(_get_body(fun))

   quote
      let c = $c
         $body
     end
  end
end

[thautwarm]

related: cscherrer/Soss.jl#321

[thautwarm]

This can be also addressed by JuliaStaging/JuliaVariables.jl#27