asg/mem-alloc: Use doubly linked-list and update checker

Add a pointer to prev in struct block_meta and move unmodifiable
files to utils/.
Add memory leak checks and style checks to the checker.
Add make check-fast option to skip checking for memory leaks.
Add instructions for debugging with the VSCode debugger.

Signed-off-by: Alex Apostolescu <alexx.apostolescu@gmail.com>