Add time-only and time-value functions to complement time-now

[blerner]

We have time-now, that returns the current time in milliseconds since the epoch. We should add

time-only :: <T> (( -> T) -> Number)
time-value :: <T> ( -> T) -> {T; Number}

fun time-only(f) block:
  start = time-now()
  f()
  stop = time-now()
  stop - start
end

fun time-value(f) block:
  start = time-now()
  ans = f()
  stop = time-now()
  { ans ; stop - start }
end

that are more usable/informative.

[shriram]

I would put a contract on these so that if someone were to accidentally pass in a non-thunk value they'll get a run-time error. I imagine a lot of students will write something like

time-only(some-big-computation())

and…not at all get what they expect. Of course they'll get an error at f() but a contract error may be better. (Ideally, they get an even more informative error that educates them, but that requires reflecting on value types and arity and what not.)

[shriram]

I might swap the order of values in time-value so that time comes first and value comes second, both matching the focus and the function's naem.

[blerner]

The first two lines of the code-snippet are contracts :)

I kept the order that (I think) @schanzer requested in the planning doc; if you want to swap it, that's trivial and fine by me.

[blerner]

Where should these functions live? In a helper library? In essentials####? Somewhere else? time-now is defined by runtime itself, since it's a JS primitive, but these others are Pyret utility functions.

[jpolitz]

Hm. I think I'd make a new module called timing. Re-export time-now and define and export these two?