Instead of manually walking the (kernel) stack inside the eBPF code,
create a `BPF_STACK_TRACE` table and store the stack IDs in the alloc
info struct.

This greatly simplifies the leak detection code: instead of storing the
full stack trace as a string to de-duplicate the allocation point for
the different allocations, we can store the `stack_id`. Since we're
creating stack IDs with `BPF_F_REUSE_STACKID`, the kernel will take care
of deduplication for us.

Additionally, the `StackDecoder` class has been specialized into a
`UStackDecoder` and `KStackDecoder` (for userland and kernel stacks,
respectively). This lets us pass the decode class directly to
`stack_traces.walk()` to automate symbol resolution.

A new class, `Allocation` has been created to encapsulate what
previously was a 2-element tuple of `(count, size)`. This