subset HVAR

Re-implemented & repurposed CFF:remap_t as hb_map2_t (moved to hb-ot-layout-common.hh) for two-way mapping for use by index map subsetting.
Hooked up HVAR subsetter through _subset2.
Some renaming in CFF code.
diff --git a/src/hb-subset.cc b/src/hb-subset.cc
index 135265f..fa6e4d9 100644
--- a/src/hb-subset.cc
+++ b/src/hb-subset.cc
@@ -45,6 +45,7 @@
 #include "hb-ot-vorg-table.hh"
 #include "hb-ot-layout-gsub-table.hh"
 #include "hb-ot-layout-gpos-table.hh"
+#include "hb-ot-var-hvar-table.hh"
 
 
 static unsigned int
@@ -198,6 +199,9 @@
     case HB_OT_TAG_GPOS:
       result = _subset2<const OT::GPOS> (plan);
       break;
+    case HB_OT_TAG_HVAR:
+      result = _subset2<const OT::HVAR> (plan);
+      break;
 
     default:
       hb_blob_t *source_table = hb_face_reference_table (plan->source, tag);