Userspace time measurement

[0xd34df00d]

I'm only replicating the behavior of timeItT. Replicating other functions seems unreasonable (looks like it's not a particularly widely needed feature, so I'm not sure cluttering the API is worth it), and changing the API to unify total CPU time vs userspace time (and maybe vs kernel time) is probably not worthy for the same reasons (compatibility seems to be more important than this feature).

[merijn]

Sorry it took so long to take a look at this, but things have been a bit hectic. Mostly looks fine, I'll probably first bump the current Travis tests to the latest GHC, then merge this later today/tomorrow.

[merijn]

Sorry, for the slowness, but I've been distracted by the corona lockdown. I just rebased the PR onto updated Travis tests and updated the System.GetRUsage module to be cleaner and compile without errors on older GHCs.

Initially I was going to merge this as-is, but in the interest of avoiding future breaking refactorings I think I'd like to be a bit more systematic. Having thought about it a bit I don't really like having timeItTWith, timeItT, and timeItUserSpaceT separately.

The idea behind timeItT was to have it be the "primitive" version on which everything was based, and having two separate ones is rather ugly. I'd prefer having a single timeItT that computes both userspace CPU time, kernel CPU time (and possibly wallclock) and then project specialised version from there.

But then using getCPUTime as fallback won't work, and simply assuming getrusage exists wouldn't work on Windows unless I find out how to compute the same thing there.