GA_DEEP_FREE_PTR: deep free macro for garrays that store simple pointers

By "simple pointer" I mean a pointer that can be freed with a call to `free`
without leaking any member pointer.

This macro does exactly what `ga_clear_strings` does.

src/nvim/garray.c

@@ -37,10 +37,7 @@ void ga_clear(garray_T *gap)
 /// @param gap
 void ga_clear_strings(garray_T *gap)
 {
-  for (int i = 0; i < gap->ga_len; ++i) {
-    free(((char_u **)(gap->ga_data))[i]);
-  }
-  ga_clear(gap);
+  GA_DEEP_CLEAR_PTR(gap);
 }
 
 /// Initialize a growing array.