Added an arena allocator

A page is a flexibly allocated structure of bytes, with a count of the
number of bytes already allocated (used) and number of bytes available
overall (available), with a pointer to the next page, if any.

heap_t is a linked list of pages.  One may allocate a requested size
off the heap which causes one of two things:
1) Either a page already exists with enough space for the requested
size, in which case that page's pointer is used as the base for the
requested pointer
2) No pages satisfy the requested size, so a new page is allocated
which is the new end of the heap.

lib/heap.h

@@ -0,0 +1,43 @@
+/* Copyright (C) 2023 Aryadev Chavali
+
+ * You may distribute and modify this code under the terms of the
+ * GPLv2 license.  You should have received a copy of the GPLv2
+ * license with this file.  If not, please write to:
+ * aryadev@aryadevchavali.com.
+
+ * Created: 2023-11-01
+ * Author: Aryadev Chavali
+ * Description: Arena allocator
+ */
+
+#ifndef HEAP_H
+#define HEAP_H
+
+#include "./base.h"
+
+#include <stdlib.h>
+
+#define PAGE_DEFAULT_SIZE 64
+
+typedef struct Page
+{
+  struct Page *next;
+  size_t used, available;
+  byte data[];
+} page_t;
+
+page_t *page_create(size_t, page_t *);
+size_t page_space_left(page_t *);
+void page_delete(page_t *);
+
+typedef struct
+{
+  page_t *beg, *end;
+  size_t pages;
+} heap_t;
+
+void heap_create(heap_t *);
+byte *heap_allocate(heap_t *, size_t);
+void heap_stop(heap_t *);
+
+#endif