DirectoryWatcher optimisation

[9il]

No description provided.

[9il]

ref was replaced with out, because previous implementations contain dst.length = 0;

[s-ludwig]

I don't remember how exactly the current API and semantics resulted, but I think we should go for an interface that can work without allocations. An obvious way to do this for the existing API would be to add a call to dst.assumeSafeAppend(); after the dst.length = 0;. More precise semantics would only overwrite dst up the its original length.

For a new API I'd either go straight with a range based interface (would have to use a range object), or simply use a scoped callback that accepts each change - probably the latter because it usually means less boilerplate code. The Appender solution can then be implemented on top of that.