mem-pool: move reusable parts of memory pool into its own file

This moves the reusable parts of the memory pool logic used by
fast-import.c into its own file for use by other components.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mem-pool.h

@@ -0,0 +1,34 @@
+#ifndef MEM_POOL_H
+#define MEM_POOL_H
+
+struct mp_block {
+	struct mp_block *next_block;
+	char *next_free;
+	char *end;
+	uintmax_t space[FLEX_ARRAY]; /* more */
+};
+
+struct mem_pool {
+	struct mp_block *mp_block;
+
+	/*
+	 * The amount of available memory to grow the pool by.
+	 * This size does not include the overhead for the mp_block.
+	 */
+	size_t block_alloc;
+
+	/* The total amount of memory allocated by the pool. */
+	size_t pool_alloc;
+};
+
+/*
+ * Alloc memory from the mem_pool.
+ */
+void *mem_pool_alloc(struct mem_pool *pool, size_t len);
+
+/*
+ * Allocate and zero memory from the memory pool.
+ */
+void *mem_pool_calloc(struct mem_pool *pool, size_t count, size_t size);
+
+#endif