TOCTOU race in prompt redraw

[slingamn]

Using the test case from #217 (calling (*Instance).Write() concurrently with (*Instance).Readline()), with v1.5.1 on go 1.20, there is a race condition (although not a data race) where the prompt is not always redrawn correctly. Example output:

> a
received a
> b
> received b
> c
received c
> d
> received d
> e
received e
> f
received f
> g
received g
> 
received 
> 
received 
> 
received 
> 
received 
> 
> received 
> 
received 
> 
received 
> 
> received 

The lines where > is followed by text are failures to redraw the prompt.

This appears to be caused by a TOCTOU race here:

readline/operation.go

Lines 48 to 50 in 7f93d88

	if !w.t.IsReading() {
	return w.target.Write(b)
	}

Removing this fast path fixes the issue, but I'm wondering if it might have negative performance implications?