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

src/type/s2k.js

@@ -31,13 +31,16 @@
  * @module type/s2k
  */
 
-import config from '../config';
+import defaultConfig from '../config';
 import crypto from '../crypto';
 import enums from '../enums.js';
 import util from '../util.js';
 
 class S2K {
-  constructor() {
+  /**
+   * @param  {Object} config (optional) full configuration, defaults to openpgp.config
+   */
+  constructor(config = defaultConfig) {
     /** @type {module:enums.hash} */
     this.algorithm = 'sha256';
     /** @type {module:enums.s2k} */