Completely document arena's functions and make region public

If someone uses the arena functionality, they may as well get the
region functionality for free.  In particular, they may just want a
fixed bump allocator implementation which they can get here for free.

arena.h

@@ -19,16 +19,95 @@
 
 #include <stdint.h>
 
-struct Region;
+/**
+   @brief A single block of memory to be used by an arena.
+
+   @details Blocks of memory arranged in a singly linked list.
+   Each individual node is a bump allocator.
+*/
+typedef struct Region
+{
+  struct Region *next;
+  uint32_t size, capacity;
+  uint8_t bytes[];
+} region_t;
+
+/**
+   @brief Allocate a new region on the heap with requested size and a pointer to
+   the next region.
+   @details If capacity is less than REGION_DEFAULT_SIZE, capacity is set to
+   REGION_DEFAULT_SIZE.
+ */
+region_t *region_make(uint32_t capacity, region_t *next);
+
+/**
+   @brief Allocate memory of requested size on the region.
+   @details If the region cannot fit the requested size, then return NULL.
+   Otherwise return a pointer to the start of the allocated memory, incrementing
+   the region size appropriately.
+ */
+uint8_t *region_alloc_flat(region_t *region, uint32_t size);
+
+/**
+   @brief Allocate memory of requested size on the region.
+   @details Iterates through the linked list of regions to find an appropriately
+   sized region for the requested size, allocating a new region if one cannot be
+   found.  This new region will have capacity at least size *
+   REGION_CAPACITY_MULT.
+
+   Returns a pointer to the start of the allocated memory, incrementing the
+   appropriate region's size.
+ */
+uint8_t *region_alloc_rec(region_t *region, uint32_t size);
+
+/**
+   @brief Delete a region, freeing its memory.
+   @details Will free all regions following it in the linked list.
+ */
+void region_delete_rec(region_t *region);
+
 typedef struct
 {
-  struct Region *beg, *end;
+  region_t *beg, *end;
 } arena_t;
 
+/**
+   @brief Allocate memory of requested size in the arena, returning a pointer to
+   the start of it.
+   @details Uses region_alloc_rec internally to allocate the memory required.
+   arena->beg and arena->end are set appropriately for this task.
+ */
 uint8_t *arena_alloc(arena_t *arena, uint32_t size);
+
+/**
+   @brief Reallocate buffer of old_size to a buffer of new_size in the
+   arena, returning a pointer to the start of the new buffer.
+
+   @details If the pointer is not allocated in the arena, return NULL.  If the
+   pointer and old_size cover a complete region reallocate the region itself to
+   fit the newly requested size, relinking it in the linked list.  Otherwise,
+   allocate as per usual.
+
+   The contents of the old memory are copied into the new buffer.  If old_size >
+   new_size, only new_size bytes will be copied from the old buffer into the new
+   one.
+ */
 uint8_t *arena_realloc(arena_t *arena, uint8_t *pointer, uint32_t old_size,
                        uint32_t new_size);
+
+/**
+   @brief Reset the arena for reuse.
+
+   @details Sets all regions to default values, setting size to 0.  No memory is
+   deleted in this operation.
+ */
 void arena_reset(arena_t *arena);
+
+/**
+   @brief Free the memory associated with the arena.
+
+   @details Deletes all regions of memory associated in the arena.
+ */
 void arena_free(arena_t *arena);
 
 #ifdef ARENA_IMPL
@@ -45,19 +124,6 @@ void arena_free(arena_t *arena);
 #endif
 
 #define MAX(A, B) ((A) > (B) ? (A) : (B))
-
-/**
-   @brief A single block of memory to be used by an arena.
-
-   @details Blocks of memory arranged in a singly linked list.
-   Each individual node is a bump allocator.
-*/
-typedef struct Region
-{
-  struct Region *next;
-  uint32_t size, capacity;
-  uint8_t bytes[];
-} region_t;
 #define MIN(A, B) ((A) < (B) ? (A) : (B))
 
 region_t *region_make(uint32_t capacity, region_t *next)