Add config parameter to top-level functions (#1241)

Refactor functions to take the configuration as a parameter.

This allows setting a config option for a single function call, whereas
setting `openpgp.config` could lead to concurrency-related issues when
multiple async function calls are made at the same time.

`openpgp.config` is used as default for unset config values in top-level
functions.
`openpgp.config` is used as default config object in low-level functions
(i.e., when calling a low-level function, it may be required to pass
`{ ...openpgp.config, modifiedConfig: modifiedValue }`).

Also,

- remove `config.rsaBlinding`: blinding is now always applied to RSA decryption
- remove `config.debug`: debugging mode can be enabled by setting
  `process.env.NODE_ENV = 'development'`
- remove `config.useNative`: native crypto is always used when available

test/general/brainpool.js

@@ -244,13 +244,13 @@ EJ4QcD/oQ6x1M/8X/iKQCtxZP8RnlrbH7ExkNON5s5g=
   });
   it('Decrypt and verify message with leading zero in hash signed with old elliptic algorithm', async function () {
     //this test would not work with nodeCrypto, since message is signed with leading zero stripped from the hash
-    const useNative = openpgp.config.useNative;
-    openpgp.config.useNative = false;
+    if (util.getNodeCrypto()) {
+      this.skip(); // eslint-disable-line no-invalid-this
+    }
     const juliet = await load_priv_key('juliet');
     const romeo = await load_pub_key('romeo');
     const msg = await openpgp.readMessage({ armoredMessage: data.romeo.message_encrypted_with_leading_zero_in_hash_signed_by_elliptic_with_old_implementation });
     const result = await openpgp.decrypt({ privateKeys: juliet, publicKeys: [romeo], message: msg });
-    openpgp.config.useNative = useNative;
     expect(result).to.exist;
     expect(result.data).to.equal(data.romeo.message_with_leading_zero_in_hash_old_elliptic_implementation);
     expect(result.signatures).to.have.length(1);