program: Port from SPL

[joncinque]

Problem

It's time to move over the SPL Slashing program implementation to its specific repo, but it hasn't been done.

Summary of changes

Following the PR for the token program at solana-program/token#12, I ran:

# add the remote for the source repo
git remote add spl https://github.com/solana-labs/solana-program-library.git
# fetch the master from spl under a different local branch
git fetch spl master:fromspl
git switch fromspl
# be safe, remove the remote
git remote rm spl
# filter the stake related files
python3 ../../git-filter-repo \
  --path slashing/program \
  --path-rename slashing/program:program \
  --force
# re-add this repo
git remote add origin [email protected]:joncinque/slashing.git
git fetch origin main
git switch main
git switch fromspl
git rebase main

NOTE: CI is not included yet

cc @AshwinSekar