refactor: spread across files

builtins.c

@@ -0,0 +1,22 @@
+#include "lisp.h"
+
+Node *lisp_function_addition(Nodes *args) {
+    if (args->count != 2) {
+        printf("'+' expects two arguments!\n");
+        return NULL;
+    }
+
+    if (args->items[0].kind != NODE_KIND_NUMBER ||
+        args->items[1].kind != NODE_KIND_NUMBER) {
+        printf("'+' can only add numbers, you specified something else!\n");
+        return NULL;
+    }
+
+    Node *node = malloc(sizeof(Node));
+    node->kind = NODE_KIND_NUMBER;
+
+    intptr_t a = args->items[0].as_number;
+    intptr_t b = args->items[1].as_number;
+    node->as_number = a + b;
+    return node;
+}