std.digest: can we get rid of start() functions?

[Piotr Szturmaj]

H. S. Teoh wrote:
> Then you could just write:
>
> 	SHA1 sha1;
> 	// calculate hash
> 	sha1 = SHA1.init;
> 	// calculate hash
> 	...
>

In my proposal reset() does exactly the same.