Implement add, sub, mul, div, inv, neg

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/util/array.h

@@ -175,6 +175,7 @@ public:
         return m_data + size(); 
     }
 
+    T const * c_ptr() const { return m_data; }
     T * c_ptr() { return m_data; }
 
     void swap(array & other) {

src/util/ref_buffer.h

@@ -78,11 +78,7 @@ public:
         return m_buffer.c_ptr();
     }
 
-    T const * operator[](unsigned idx) const {
-        return m_buffer[idx];
-    }
-
-    T * operator[](unsigned idx) {
+    T * operator[](unsigned idx) const {
         return m_buffer[idx];
     }
 
@@ -144,6 +140,7 @@ public:
             return *this;
         reset();
         append(other);
+        return *this;
     }
 };