Function record

record(buffer: number): () => SoundPromise
Records a sound until the returned stop function is called. Takes a buffer duration (in seconds) as argument, and returns a nullary stop. A call to the stop function returns a Sound promise.

How the function behaves in detail:
1. record is called.
2. The function waits for the given buffer duration.
3. The recording signal is played.
4. Recording begins when the recording signal finishes.
5. Recording stops when the returned stop function is called.
Parameters
- buffer: number
  pause before recording, in seconds
Returns () => SoundPromise
Example
```
init_record();
const stop = record(0.5); // record after 0.5 seconds.
const promise = stop();   // stop recording and get sound promoie
const sound = promise();  // retrieve the recorded sound
play(sound);              // and do whatever with it
```