lib/strings: hide asciiTable

Since it's an attribute set, the lib function location generating code
tries to generate locations for each of the characters...

lib/strings.nix

@@ -4,6 +4,8 @@ let
 
 inherit (builtins) length;
 
+asciiTable = import ./ascii-table.nix;
+
 in
 
 rec {
@@ -34,8 +36,6 @@ rec {
     unsafeDiscardStringContext
     ;
 
-  asciiTable = import ./ascii-table.nix;
-
   /* Concatenate a list of strings.
 
     Type: concatStrings :: [string] -> string