Provide a DAG interface

[thufschmitt]

A number of things in the final environment are lists, and lists don't compose well (they don't compose at all currently in Nickel, and that's arguably the less bad solution). As a consequence, we'll likely want to internally represent them as DAGs. Defining a DAG is easy (see below), but fairly verbose to use.

With a smart contract (no pun intended), we could probably make this much lighter by just normalizing the fields that we pass in.

let
    DagElement = fun Data => {
            before : Array Str | default = [],
            after : Array Str | default = [],
            value : Data
        }
in let
    Dag = fun Data => {
        _: DagElement Data
    }
in
let
  # FIXME: This isn't sorting anything, would have to
  topo_sort | Dag Str -> Array Str = fun dag => array.map (fun elt => elt.value) (record.values dag)
in
let StructuredEnvVar = { sep : Str | default = ":", elems : Dag Str } in
let StructuredEnv = { _ : StructuredEnvVar } in
let Env = { _ : Str } in
let destructure_env_elt | Str -> StructuredEnvVar -> Str = fun name { sep, elems } => string.join sep (topo_sort elems) in
let destructure_env | StructuredEnv -> Env = fun env => record.map destructure_env_elt env in
{
    env | Env = destructure_env structured_env,

    # Building the DAG in client code. This is so nice because it composes well,
    # but it's also quite heavy to write
    structured_env | StructuredEnv = {
      PATH.elems = {
        bash.value = "$bash/bin",
        gcc.value = "$gcc/bin",
        clang.value = "$clang/bin",
        clang.before = ["gcc"],
      },
    },

    # Would be nice to be able to just write (for the simple cases)
    # something like
    # ```
    # structured_env.PATH = [ "$bash/bin", "$clang/bin", "$gcc/bin" ],
    # ```
    # which would expand to
    # ```
    # Structured_env.PATH.elems = {
    #  "$bash/bin" = { value = "$bash/bin", before = ["$clang/bin"], },
    #  "$clang/bin" = { value = "$clang/bin", before = ["$gcc/bin"], },
    #  "$gcc/bin" = { value = "$gcc/bin", },
    # },
}