rename uc_context_free() to uc_mem_free(). see #373

tests/regress/mem_64_c.c

@@ -32,7 +32,7 @@ int main(int argc, char **argv, char **envp) {
                     starts[i], regions[i].begin);
          }
       }
-      free(regions);
+      uc_mem_free(regions);
    }
    
    uc_close(uc);

tests/unit/test_x86.c

@@ -800,7 +800,7 @@ static void test_i386_reg_save(void **state)
     assert_int_equal(eax, 2);
 
     // clean up;
-    uc_context_free(saved_context);
+    uc_mem_free(saved_context);
     uc_assert_success(uc_close(uc));
 }
 /******************************************************************************/