Rust crypto mini warts

Amalgamated issue so I don't forget about it.
- `Buffer` is used liberally assuming it exists when it practice it may not (in browsers without polyfills). `../matrix-js-sdk/src/rust-crypto/rust-crypto.ts:1073: return Buffer.from(backupKeys.decryptionKey.toBase64(), "base64");` - Solution: use the util class which handles this for us `encodeBase64`.
- Type definitions on `export interface CreateSecretStorageOpts ` imply all args are optional but `createSecretStorageKey` is not, it is required.
- Rust crypto calls `keyFromPassphrase` https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/crypto/key_passphrase.ts#L55 which does a `global.Olm` check and fails if it isn't found, meaning rust crypto relies on old crypto..

Start with src/rust-crypto/rust-crypto.ts:1073 and the CreateSecretStorageOpts definition, then read src/crypto/key_passphrase.ts at line 55. Check how the browser Buffer use, required secret-storage key, and global.Olm dependency behave. Done means each of the three listed Rust crypto concerns is addressed without relying on unavailable browser globals.

Written by the indexing model from the issue text.