Allow primops to have Markdown documentation

src/libexpr/eval.hh

@@ -30,6 +30,8 @@ struct PrimOp
     PrimOpFun fun;
     size_t arity;
     Symbol name;
+    std::vector<std::string> args;
+    const char * doc = nullptr;
 };
 
 
@@ -240,6 +242,8 @@ private:
     Value * addPrimOp(const string & name,
         size_t arity, PrimOpFun primOp);
 
+    Value * addPrimOp(PrimOp && primOp);
+
 public:
 
     Value & getBuiltin(const string & name);