* Specification does not permit other hash algorithms
  for RSA signatures

* Avoid Class tests, easier to add EC keys later (also asym)

* Create a "Key" with only a certificate (for verification)
* Create a Key with a private key for signing/authorising
* Create a Key with both because someone will want to
* Generate exception if certificate and key are not related
* Separate signing flows for hmac (secret) and asymmetric (privateKey)

Resolves 99designs#35 (comment)

* Unit Test for bad algorithm choice
* Common algorithm selection function to improve maintainability
  when algorithms change (single implementation in verify() and sign()
* Move Algorithm instantiation to Context creation:
  Get earlier feedback on bad
  hash algorithm choice - error should occur at time of bad
  data input, not usage.

* Need better documentation around key handling (e.g. no mention
  of using an existing KeyStore object)

* Hide internal representation of keys
* Better approach for asymmetric keys in one Key object
* function getSigningKey() - Secret for HMAC, Private Key for RSA
* function getVerifyingKey() - Secret for HMAX, Piblic Key for RSA
* Move from certificates to public keys for overall consistency
* Preserve existing key import methods (may need to change with future developments)

* Implementation is correct against this draft, good to use
  latest link

* Key now handles inputs as X.509, naked Public Key and
  Private Key (all PEM)
* Improved & clearer detection logic with explicit tests
  for asymmetric key material, falling back on shared secret
  for HMAC if asymmetric keys are not detected
  hasX509Certificate()
  hasPublicKey()
  hasPrivateKey()
* Clearer names for test constants (easier to expand to EC later)

- Remove unsupported Symfony/PHP version combinations
- Add PHP 7.3 support