feat(json): emit JSON with correct control-character escaping

So generated output survives paths containing spaces, quotes or backslashes.

lib/json.ml

@@ -0,0 +1,17 @@
+let escape s =
+  let b = Buffer.create (String.length s + 8) in
+  String.iter
+    (fun c ->
+      match c with
+      | '"' -> Buffer.add_string b "\\\""
+      | '\\' -> Buffer.add_string b "\\\\"
+      | '\n' -> Buffer.add_string b "\\n"
+      | '\t' -> Buffer.add_string b "\\t"
+      | '\r' -> Buffer.add_string b "\\r"
+      | c when Char.code c < 0x20 -> Buffer.add_string b (Printf.sprintf "\\u%04x" (Char.code c))
+      | c -> Buffer.add_char b c)
+    s;
+  Buffer.contents b
+
+let str s = "\"" ^ escape s ^ "\""
+let list f l = "[" ^ String.concat ", " (List.map f l) ^ "]"