Some valid Path values cannot be applied to their source Value

[jbardin]

A Value containing a set cannot use the path argument within a Transform callback for all paths.

Given an object like so:

ObjectVal(map[string]Value{
  "set": SetVal([]Value{
    ObjectVal(map[string]Value{
      "attr": StringVal("val"),
    }),
  }),
})

Using Transform to obtain each step's value from another object will encounter a path which cannot be used.

// using the same value to ensure all paths are valid, and should result in the same value
Transform(val, func(p Path, v Value) (Value, error) {
    return p.Apply(val)
})

This results in at step 1: key value not number or string when the path is for

cty.Path{cty.GetAttrStep{Name:"set"}, cty.IndexStep{Key:cty.ObjectVal(map[string]cty.Value{"attr":cty.StringVal("val")})}, cty.GetAttrStep{Name:"attr"}}

I'm not sure how we would intend to handle this. Sets are not currently index-able, so simply skipping the key value not number or string check doesn't work. Should we make all Path values valid for Apply on a correctly shaped Value, have special handling to detect when a set is encountered in the path, or maybe just document this as a potential hazard within Transform and Walk?