Rework tree generation mode and API

Store tree nodes in congruent, compact arena array.
Define handle types to refer to tree nodes rather than pointers to
structure instances.
Free tree with context.

spec/test_tree_node_memory_remains.d

@@ -378,19 +378,19 @@ def main() -> int
     context = p_context_new(input);
     size_t result = p_parse(context);
     assert_eq(P_SUCCESS, result);
-    PModule * pmod = p_result(context);
-    PModuleItems * pmis = pmod.pModuleItems;
-    PFunctionDefinition *[] pfds;
-    while (pmis !is null)
+    PModule pmod = p_result(context);
+    PModuleItems pmis = pmod.pModuleItems;
+    PFunctionDefinition[] pfds;
+    while (pmis.valid)
     {
-        PModuleItem * pmi = pmis.pModuleItem;
-        if (pmi is null)
+        PModuleItem pmi = pmis.pModuleItem;
+        if (!pmi.valid)
         {
             stderr.writeln("pmi is null!!!?");
             assert(0);
         }
-        PFunctionDefinition * pfd = pmi.pFunctionDefinition;
-        if (pfd !is null)
+        PFunctionDefinition pfd = pmi.pFunctionDefinition;
+        if (pfd.valid)
         {
             pfds = [pfd] ~ pfds;
         }
@@ -405,5 +405,5 @@ def main() -> int
             stderr.writeln("Index ", i, ": expected ", expected[i].name, "/", expected[i].token, ", got ", pfds[i].name.pvalue.s, "/", pfds[i].returntype.pType.pTypeBase.pToken1.token);
         }
     }
-    p_tree_delete(pmod);
+    p_context_delete(context);
 }