resolves #134

Fixes RangeError in base64url.ts when encrypting large Uint8Arrays

String.fromCharCode.apply causes a RangeError for large Uint8Arrays
(> ~500kB). This happens, e.g., when encrypting larger files.

See this gist to reproduce the bug (select a large file and see the
browser console):
https://gist.github.com/codedust/88c8af3b2acd782e72ffbe0c3c8bf5af

Error message in Firefox:
```
Uncaught (in promise) RangeError: too many arguments provided for a
function call (in base64url.js:8:62)
    encode https://localhost:8000/jose/runtime/base64url.js:8
    encrypt https://localhost:8000/jose/jwe/flattened/encrypt.js:143
```

Error message in Chromium:
```
Uncaught (in promise) RangeError: Maximum call stack size exceeded
    at encode (base64url.js:8)
    at FlattenedEncrypt.encrypt (encrypt.js:143)
    at async CompactEncrypt.encrypt (encrypt.js:23)
    at async jwe_test ((index):55)
```

Solution: Apply String.fromCharCode.apply in chunks of 32768 bytes,
see https://stackoverflow.com/a/12713326