Add tokenlist, a linked list of lexer tokens

The linked list is doubly linked so the parser can look forward into it
and error reporting can look backward.

This commmit also reworks main to use the tokenlist instead of dealing
with the lexer manually.

src/tokenlist.c

@@ -0,0 +1,83 @@
+#include "tokenlist.h"
+#include "error.h"
+#include "lexer.h"
+#include <stdlib.h>
+
+error_t *tokenlist_alloc(tokenlist_t **output) {
+    *output = nullptr;
+
+    tokenlist_t *list = calloc(1, sizeof(tokenlist_t));
+    if (list == nullptr)
+        return err_allocation_failed;
+
+    list->head = nullptr;
+    list->tail = nullptr;
+
+    *output = list;
+    return nullptr;
+}
+
+error_t *tokenlist_entry_alloc(tokenlist_entry_t **output) {
+    *output = nullptr;
+
+    tokenlist_entry_t *entry = calloc(1, sizeof(tokenlist_entry_t));
+    if (entry == nullptr)
+        return err_allocation_failed;
+
+    entry->next = nullptr;
+    entry->prev = nullptr;
+
+    *output = entry;
+    return nullptr;
+}
+
+void tokenlist_append(tokenlist_t *list, tokenlist_entry_t *entry) {
+    if (list->head == nullptr) {
+        list->head = entry;
+        list->tail = entry;
+        entry->next = nullptr;
+        entry->prev = nullptr;
+    } else {
+        entry->prev = list->tail;
+        entry->next = nullptr;
+        list->tail->next = entry;
+        list->tail = entry;
+    }
+}
+
+void tokenlist_entry_free(tokenlist_entry_t *entry) {
+    lexer_token_cleanup(&entry->token);
+    free(entry);
+}
+
+void tokenlist_free(tokenlist_t *list) {
+    if (list == nullptr)
+        return;
+
+    tokenlist_entry_t *current = list->head;
+    while (current) {
+        tokenlist_entry_t *next = current->next;
+        tokenlist_entry_free(current);
+        current = next;
+    }
+
+    free(list);
+}
+
+error_t *tokenlist_fill(tokenlist_t *list, lexer_t *lex) {
+    error_t *err = nullptr;
+    lexer_token_t token = {};
+    while ((err = lexer_next(lex, &token)) == nullptr) {
+        tokenlist_entry_t *entry;
+        err = tokenlist_entry_alloc(&entry);
+        if (err) {
+            lexer_token_cleanup(&token);
+            return err;
+        }
+        entry->token = token;
+        tokenlist_append(list, entry);
+    }
+    if (err != err_eof)
+        return err;
+    return nullptr;
+}