Add if statements, unary minus, and fix GC safe points

- Lexer: recognize 'if' as keyword (TOK_IF)
- Parser: add NODE_IF with if_statement union, parse if/cond/body,
  handle unary minus in parse_term as 0 - expr
- Eval: add NODE_IF evaluation, move GC to NODE_BLOCK level to avoid
  destroying temporary values during sub-expression evaluation

src/vm/eval.h

@@ -41,16 +41,8 @@ void env_set(Environment *env, const char *name, size_t value) {
 
 int step = 0;
 
-size_t eval(ASTNode *node, Environment *env, void *allocator, int debug, int gc) {
-
-  // Run GC
-  if (gc) {
-    size_t roots[256];
-    for (int i = 0; i < env->count; i++) {
-      roots[i] = env->vars[i].value;
-    }
-    gc_collect(allocator, roots, env->count);
-  }
+size_t eval(ASTNode *node, Environment *env, void *allocator, int debug,
+            int gc) {
 
   if (debug > 0) {
     step++;
@@ -101,6 +93,15 @@ size_t eval(ASTNode *node, Environment *env, void *allocator, int debug, int gc)
     return val;
   }
   case NODE_BLOCK:
+    // Run GC
+    if (gc) {
+      size_t roots[256];
+      for (int i = 0; i < env->count; i++) {
+        roots[i] = env->vars[i].value;
+      }
+      gc_collect(allocator, roots, env->count);
+    }
+    
     for (int i = 0; i < node->data.block.count; i++)
       eval(node->data.block.stmts[i], env, allocator, debug, gc);
     return 0;