[FEATURE] eve::compress_copy, eve::algo::copy_if return type issue

[DenisYaroshevskiy]

Because we need to do unsafe stores, we need to know the output size.
This leads to interface of eve::algo::copy_if(in_range, out_range, predicate).

So - what should happen if out_range doesn't have enough space to store all of the selected elements? We should stop and return where we stopped: .in = after_the_last_written, .out = out_range.end()

However, in versions of compress_copy that use compress_store computing after_the_last_written seems expensive.
Would be nice to come up with a way to return it though.

[DenisYaroshevskiy]

Another way of explaining the issue:

auto x = eve::load(in);
auto m = p(x);
out = eve::compress_store[safe][ignore_out](x, m, out);
in += (?)

If ignore_out == ignore_none or we wrote all the elements - (?) => x.size();
But if it's not, then it becomes nth_true(m, out - old_out)