= Nix expression support

When `(nix reader)` is imported, it is possible to execute Nix code inline with
Scheme code. Simply wrap your Nix code in curly brackets:

[,scheme]
----
(write
  (string-append
    "Hello, Nix version"
    { builtins.nixVersion }))
----

The following values can be translated:

|===
| Nix            | Scheme  | Notes

| string         | string | (Loses string context.)
| integer        | number |
| float          | number |
| boolean        | boolean | 
| lambda         | procedure | (with single argument)
| list           | vector or list | Depends on the status of `\*translate-list-as-vector*`
| attrset        | alist |
| builtin        | procedure |
| external value | symbol, other unknown objects |
|===

If a value in Nix is preceded with a comma, it is unquoted, similar to
`(unquote)` in a Scheme quasiquotation. If prefixed with a single quote, it is
`(quote)`-d.

[,scheme]
----
(define
  (test-append foo)
  (string-append "Hello, " foo))
(write
  { ,test-append "world!") })
----