Track AST nodes' sizes in characters as well as bytes

The `pos` and `size` functions for Nodes now return TSLength structs,
which contain lengths in both characters and bytes. This is important
for knowing the number of unicode characters in a Node.

spec/runtime/stack_spec.cc

@@ -27,7 +27,12 @@ describe("stacks", [&]() {
     TSTree *node1;
 
     before_each([&]() {
-      node1 = ts_tree_make_leaf(sym1, 5, 1, 0);
+      node1 = ts_tree_make_leaf(
+          sym1,
+          (TSLength) { 0, 0 },
+          (TSLength) { 1, 1 },
+          0);
+
       ts_stack_push(&stack, 5, node1);
     });