The Ancient Rite of Passage: Writing your own memory manager in plain C ;).

I wrote this following this excellent online tutorial: https://danluu.com/malloc-tutorial/.

I've slightly refactored the code, annotated it extensively, and also improved it a bit (at least, I think I did...).

• Added a best-fit search as an alternative to the current first-fit search in find_free_block().
• Introduced pointer TAIL to last element of the global linked list of memory blocks, so that we don't have to search for it anew every time. In my opinion, this makes the code slightly easier to understand, as we avoid the double pointer in the parameters of find_free_block() that gets changed during search.
• Removed sbrk(0) before sbrk(META_SIZE + size) in request_space(), because as far as I can tell this is not required.
• Switched to a double-linked list instead of a single-linked one to make finding the left neighbour happen in O(1) instead of O(n).
• Added a convenience function to print the current state of the global linked list of memory blocks.
• Added block splitting when a free block is recycled which is too large (has been suggested as additional exercise in the tutorial).
• Added block merging upon freeing of blocks (has been suggested as additional exercise in the tutorial).
• Added overflow check in calloc() (has been suggested as additional exercise in the tutorial).