Strictness of modify in Control.Monad.State.Strict

[Evan Laforge]

> In general it would be nice if the notion of strictness were better
> addressed in the major libraries' documentation. It's a nuanced
> issue which is not fully captured by a ".Lazy" or ".Strict" in the
> module name.

I agree.  I too wasted a significant amount of time hunting down space
leaks with were fixed by adding a modify' function.  If it had been in
the State module along with documentation about the implications, I
could have saved time.