give hybrid meaning to group_indices(), as in #1185

```r
> df <- data_frame(v1 = c(NA, NA, 2, 2, 3), v2 = c(NA, NA, 3,3, 4)) %>% group_by(v1)
> df %>% mutate( g = group_indices() )
# A tibble: 5 x 3
# Groups:   v1 [3]
     v1    v2     g
  <dbl> <dbl> <int>
1   NA    NA      3
2   NA    NA      3
3    2.    3.     1
4    2.    3.     1
5    3.    4.     2
```

inst/include/dplyr/HybridHandlerMap.h

@@ -14,6 +14,7 @@ void install_window_handlers(HybridHandlerMap& handlers);
 void install_offset_handlers(HybridHandlerMap& handlers);
 void install_in_handlers(HybridHandlerMap& handlers);
 void install_debug_handlers(HybridHandlerMap& handlers);
+void install_group_handlers(HybridHandlerMap& handlers);
 
 bool hybridable(RObject arg);
 

src/hybrid.cpp

@@ -72,6 +72,7 @@ HybridHandlerMap& get_handlers() {
     install_offset_handlers(handlers);
     install_in_handlers(handlers);
     install_debug_handlers(handlers);
+    install_group_handlers(handlers);
   }
   return handlers;
 }
@@ -228,7 +229,7 @@ Result* get_handler(SEXP call, const ILazySubsets& subsets, const Environment& e
 
   if (TYPEOF(call) == LANGSXP) {
     int depth = Rf_length(call);
-
+    
     HybridHandlerMap& handlers = get_handlers();
 
     bool in_dplyr_namespace = false;

src/hybrid_group.cpp

@@ -0,0 +1,46 @@
+#include "pch.h"
+#include <dplyr/main.h>
+
+#include <dplyr/HybridHandlerMap.h>
+
+#include <dplyr/Result/ILazySubsets.h>
+
+#include <dplyr/Result/Result.h>
+
+using namespace Rcpp;
+using namespace dplyr;
+
+
+class GroupHybrid : public Result {
+public:
+  SEXP process(const RowwiseDataFrame&) {
+    Rprintf( "GroupHybrid::RowwiseDataFrame()\n" ) ;
+    return R_NilValue ;
+  }
+
+  SEXP process(const GroupedDataFrame&) {
+    Rprintf( "GroupHybrid::GroupedDataFrame()\n" ) ;
+    return R_NilValue ;
+  }
+
+  SEXP process(const FullDataFrame&) {
+    Rprintf( "GroupHybrid::FullDataFrame()\n" ) ;
+    return R_NilValue ;
+  }
+
+  SEXP process(const SlicingIndex& i) {
+    return IntegerVector(i.size(), i.group() + 1);
+  }
+};
+
+Result* group_prototype(SEXP call, const ILazySubsets&, int nargs) {
+  if (nargs != 0)
+    return 0;
+
+  return new GroupHybrid();
+}
+
+void install_group_handlers(HybridHandlerMap& handlers) {
+  Environment ns_dplyr = Environment::namespace_env("dplyr");
+  handlers[ Rf_install("group_indices") ] = HybridHandler(group_prototype, ns_dplyr["group_indices"]);
+}