Increase size of ref_counts and add assertions

This bumps the size of the reference counts from 16- to 32-bit counters to make
it less likely to overflow. Also assert in the retain function that the
reference count didn't overflow.

32-bits seems big enough for non-pathological examples but a more fool-proof
fix may be to bump it to 64-bits.

src/runtime/tree.c

@@ -232,6 +232,7 @@ Tree *ts_tree_make_error_node(TreeArray *children) {
 void ts_tree_retain(Tree *self) {
   assert(self->ref_count > 0);
   self->ref_count++;
+  assert(self->ref_count != 0);
 }
 
 void ts_tree_release(Tree *self) {