Add optional description to @time and new @showtime macro

[IanButterworth]

Adds an optional description to @time and @timev, and a new @showtime macro that is supposed to mimic the way @show prints the expr being evaluated first.

Intends to optionally make it a little easier to associate time reports with their source when the macros are used in multiple places

julia> @time sleep(0.2)
  0.204988 seconds (5 allocations: 144 bytes)

julia> @time "Sleep for a bit" sleep(0.2)
Sleep for a bit ->  0.203607 seconds (5 allocations: 144 bytes)

julia> for loop in 1:3
			@time loop sleep(0.2)
		end
1 ->  0.205158 seconds (5 allocations: 144 bytes)
2 ->  0.203381 seconds (5 allocations: 144 bytes)
3 ->  0.203496 seconds (5 allocations: 144 bytes)

julia> function foo() # a function where you don't know which order the @time's will print
		@sync begin
			  @async @time "first async" sleep(rand())
			  @async @time "second async" sleep(rand())
		end
		return
	   end

julia> foo()
second async ->  0.797018 seconds (5 allocations: 144 bytes)
first async ->  0.902320 seconds (12.01 k allocations: 577.507 KiB, 1.49% compilation time)

julia> @showtime sleep(0.2)
sleep(0.2) ->  0.205479 seconds (5 allocations: 144 bytes)

Also, there might be a better print format than message -> 0.797018 seconds (5 allocations: 144 bytes) but I couldn't think of one that wouldn't be more confusing

[IanButterworth]

Maybe @showtime isn't quite the right name? It does mimic the LHS of @show, but it doesn't show the object value, so that might be confusing?

@reprtime?

Edit: I think it's fine.. It's like @show but for @time

[JeffBezanson]

[timholy]

If we're expanding the @show family...should we add @showln? https://github.com/timholy/DebuggingUtilities.jl#showln
It's not widely used, I think, but it's perhaps because you actually have to make that package a dependency of whatever you're debugging. If it's in Base, problem goes away.

We could also make it @show line=true expr and save an export.

[JeffBezanson]

I think the arrow (->) is a bit strange here; maybe a colon instead?

[IanButterworth]

Like this? Note that I also removed a 2nd lpad space on the time output after the :

julia> @showtime repeat("beetlejuice", 2)
repeat("beetlejuice", 2) : 0.000002 seconds (1 allocation: 40 bytes)
"beetlejuicebeetlejuice"

[IanButterworth]

@JeffBezanson @fredrikekre if the above looks good to you I'll PR it

[IanButterworth]

Formatting updated in #43255