stimes implementations

[sjakobi]

I noticed that the Semigroup instances don't explicitly define stimes. I suspect its possible to improve on the default definitions.

[Boarders]

I think we should be able to use a suitable variant of replicate (that accepts an integral argument) to improve the situation. We could also provide an implementation for sconcat that uses the specialised concat function as the default implementation probably allocates way more than needed.

[kindaro]

I shall look into it. At least I can draft some code that type checks and then we shall see if there is improvement in bench marks.

[kindaro]

And there is improvement in bench marks!