Handle nix symbolic strings on Nix side

This allows us to shortcut usage of NixString in many places.
We could probably get rid of NixString entirely as well, but it needs
more consideration.

Part of #58.

lib/contracts.ncl

@@ -14,6 +14,12 @@ let predicate | doc "Various predicates used to define contracts"
       std.is_record x
       && std.record.has_field type_field x
       && x."%{type_field}" == "nixInput",
+    is_nix_symbolic_string = fun value =>
+      std.is_record value
+      && std.record.has_field "tag" value
+      && value.tag == 'SymbolicString
+      && std.record.has_field "prefix" value
+      && value.prefix == 'nix,
     is_nix_string = fun value =>
       std.is_record value
       && std.record.has_field type_field value
@@ -150,18 +156,14 @@ from the Nix world) or a derivation defined in Nickel.
     = fun label value =>
       # A contract must always be idempotent (be a no-op if applied a second
       # time), so we accept something that is already a NixString
-      if predicate.is_nix_string value then
+      if predicate.is_nix_string value || predicate.is_nix_symbolic_string value then
         value
         # We accept a single string fragment (a plain string, a derivation or a
-        # Nix path). We normalize it by wrapping it as a one-element array
+        # Nix path). We normalize it by wrapping it into symbolic string
       else if predicate.is_string_fragment value then
-        mk_nix_string [std.contract.apply NixStringFragment label value]
+        nix-s%"%{std.contract.apply NixStringFragment label value}"%
       else
-        # TODO: it's for debugging, but we should remove the serializing at some
-        # point.
-        let label = std.contract.label.append_note (std.serialize 'Json value) label in
-        let { fragments, .. } = std.contract.apply NixSymbolicString label value in
-        mk_nix_string fragments,
+        std.contract.apply NixSymbolicString label value,
 
   NixDerivation
     | doc m%"

lib/lib.nix

@@ -94,7 +94,7 @@
             value.nix_drv);
           in
             derivation prepared
-          else if organistType == "nixString"
+          else if organistType == "nixString" || (organistType == "" && value.tag or "" == "SymbolicString" && value.prefix or "" == "nix")
           then builtins.concatStringsSep "" (builtins.map importFromNickel_ value.fragments)
           else if organistType == "nixPath"
           then baseDir + "/${value.path}"