feat(install): copy installable outputs under a prefix

Destinations come from each target's install field, the executable bit is
preserved, and a dry run prints the plan without touching anything.

lib/install.ml

@@ -0,0 +1,49 @@
+open Types
+
+let dest_dir p = function
+  | "bin" -> Filename.concat p.prefix "bin"
+  | "lib" -> Filename.concat p.prefix "lib"
+  | "include" -> Filename.concat p.prefix "include"
+  | other -> if Filename.is_relative other then Filename.concat p.prefix other else other
+
+let plan p =
+  let from_targets =
+    List.filter_map
+      (fun (t : target) ->
+        match t.install with
+        | None -> None
+        | Some where ->
+            let src = Build.out_of p t in
+            Some (src, Filename.concat (dest_dir p where) (Filename.basename src)))
+      p.targets
+  in
+  let from_blocks =
+    List.map (fun i -> (i.from, Filename.concat (dest_dir p i.dest) (Filename.basename i.from)))
+      p.installs
+  in
+  from_targets @ from_blocks
+
+let copy ~mode src dst =
+  Fs.mkdir_p (Filename.dirname dst);
+  let ic = open_in_bin src in
+  let n = in_channel_length ic in
+  let data = really_input_string ic n in
+  close_in ic;
+  let oc = open_out_gen [ Open_wronly; Open_creat; Open_trunc; Open_binary ] mode dst in
+  output_string oc data;
+  close_out oc;
+  Unix.chmod dst mode
+
+let executable src =
+  match Unix.stat src with
+  | { Unix.st_perm; _ } -> st_perm land 0o111 <> 0
+  | exception Unix.Unix_error _ -> false
+
+let execute ?(dry = false) p =
+  let items = plan p in
+  List.iter
+    (fun (src, dst) ->
+      if not (Sys.file_exists src) then Diag.error "%s has not been built yet" src;
+      if not dry then copy ~mode:(if executable src then 0o755 else 0o644) src dst)
+    items;
+  items