feat(cache): key actions on content, argv and the real input set

Digests every input plus the exact argument vector into one key, persisted
under the build directory. Staleness becomes a content question rather than a
timestamp comparison.

lib/cache.ml

@@ -0,0 +1,39 @@
+type t = { path : string; keys : (string, string) Hashtbl.t }
+
+let load path =
+  let keys = Hashtbl.create 64 in
+  (match Fs.read path with
+  | exception Sys_error _ -> ()
+  | raw ->
+      String.split_on_char '\n' raw
+      |> List.iter (fun line ->
+             match String.index_opt line '\t' with
+             | None -> ()
+             | Some i ->
+                 let key = String.sub line 0 i in
+                 let out = String.sub line (i + 1) (String.length line - i - 1) in
+                 if out <> "" then Hashtbl.replace keys out key));
+  { path; keys }
+
+let save c =
+  let b = Buffer.create 4096 in
+  Hashtbl.iter (fun out key -> Buffer.add_string b (key ^ "\t" ^ out ^ "\n")) c.keys;
+  Fs.write c.path (Buffer.contents b)
+
+let current c out = Hashtbl.find_opt c.keys out
+let put c out key = Hashtbl.replace c.keys out key
+let drop c out = Hashtbl.remove c.keys out
+
+let digest_file p = match Digest.file p with d -> Some (Digest.to_hex d) | exception Sys_error _ -> None
+
+let key ~cmd ~inputs =
+  let b = Buffer.create 256 in
+  Buffer.add_string b cmd;
+  List.iter
+    (fun p ->
+      Buffer.add_char b '\000';
+      Buffer.add_string b p;
+      Buffer.add_char b '\000';
+      Buffer.add_string b (match digest_file p with Some d -> d | None -> "?"))
+    (List.sort compare inputs);
+  Digest.to_hex (Digest.string (Buffer.contents b))