WIP: add find(first|next|prev|last) variants with nothing sentinel

[timholy]

This will allow Compat.findnext to use the new nothing sentinel.

I'm frankly not sure what to do about several of the aspects of this PR, particularly the string-related ones. I'm also not sure we need the SparseArray changes in Compat? Advice appreciated (or help from anyone who wants to take it from here).

[fredrikekre]

I'm also not sure we need the SparseArray changes in Compat

Those where added just recently and are not there on 0.6, should be fine to use the slower fallbacks on 0.6.

[fredrikekre]

AFAICT all of those returned 0 previously? Could the implementations here not simply be

if VERSION < v"0.7.0-something"
    function findnext(A, start::Integer)
        i = Base.findnext(A, start)
        if i == 0
            return nothing
        end
        return i
    end
else
    const findnext = Base.findnext
end

[timholy]

That would be a little dangerous for the day when I want to do this:

a = OffsetArray([false, true, false], -1:1)
Compat.findfirst(a)

and expect it to work properly.

[nalimilan]

Wow! Thanks for doing this radical backporting work!

However, to be honest, I'm hesitant about whether it's a good idea to copy all of these functions. @fredrikekre's proposal is appealing for its simplicity. All this duplicated code will have to be kept in sync with Base, which is tedious (I have already a few new PRs open) and which we will probably forget to do. Are you really concerned about people using the brand new OffsetArrays support on Julia 0.6? By the time users will have discovered it exists, we'll have dropped support for that release, and the error is relatively easy to catch anyway.

[garrison]

Have these methods stabilized enough in Base that they could be brought in here? I don't have a strong opinion on whether they should be copied or the simpler approach should be taken, but either way it would be great to come to a decision soon so porting can continue.

[nalimilan]

Actually this has been covered by #484 and #513.