count(itr) to count trues

[cossio]

I think it would be useful to add a method of count that takes a single iterable argument of booleans and returns the number of true elements.

For example (imagine that the function iseven does not exist), I find this:

count(i % 2 == 0 for i in 1:10)

to be more readable than this:

count(i -> i % 2 == 0, 1:10)

[cossio]

I could do a PR if someone points me to the correct source file.

[nalimilan]

You can find out where the method lives with:

julia> @which count(isodd, 1:2)
count(pred, itr) in Base at reduce.jl:654

[ararslan]

This change could be as simple as

count(itr) = count(identity, itr)

That definition matches what we do with other reduction functions.

[stevengj]

countnz(i % 2 == 0 for i in 1:10) works (as does sum), but since we already have count I agree that a one-argument version makes sense.