Add ssreflect file (from Iris).

This closes issue #177 (closed).

stdpp/ssreflect.v

+(** This file provides support for using std++ in combination with the ssreflect
+tactics. It patches up some global options of ssreflect. *)
+From Coq.ssr Require Export ssreflect.
+From stdpp Require Export prelude.
+From stdpp Require Import options.
+
+(** Restore Coq's normal "if" scope, ssr redefines it. *)
+Global Open Scope general_if_scope.
+
+(** See Coq issue #5706 *)
+Global Set SsrOldRewriteGoalsOrder.
+
+(** Overwrite ssr's [done] tactic with ours *)
+Ltac done := stdpp.tactics.done.