GitLab

- CI now pulls own composer binary instead of relying on global binary

- seal/unseal provides an easy asymmetric en & decryption, using only
  a public key to encrypt a message or file (classic asymmetric encryption).
  Therefor a signing using a private key is omitted.

- upgrades phpunit to 9.x
- migrates some typehints to php 8 union types
- refactors Key/KeyPair constructs to separated static methods

- fixes some typos
- fixes final private function warning

- updates readme with informations to utilized crypto-functions: xsalsa20 and X25519
- fixes typo in Keypair::keygen exception for invalid salt lengths

- adds file-crypto unittests

- adds ricwein/filesystem support to composer.json