Add a cheap HistoryBuffer::empty method

[Eugeny]

Currently there's no easy way to drain a HistoryBuffer without zeroing it.

A simple function like this would come in handy in performance critical situations:

fn empty(&mut self) {
  self.write_at = 0;
  self.filled = false;
}

[Dirbaio]

@Eugeny could you share a snippet of code where clear() is causing slow zeroing of the buffer, and your proposed empty() doesn't, when building with optimizations?

From some quick testing, both optimize to the same asm code.

[reitermarkus]

Closing due to no response and clear already producing the same code.