Add functions like findonly, uniqueonly, ...

[aplavin]

I'd like to propose adding a bunch of simple functions to Julia:

• findonly: complements findfirst and findlast, throws if there are multiple matches
• filterfirst, filteronly, maybe filterlast: same as first/last/only(filter(pred, collection)) but faster
• uniqueonly: like only(unique(collection)), but faster

These are straightforward to implement, and the motivation to include them is similar to that of only(): mostly for safety and convenience.

[jakobnissen]

In my opinion, findonly, filterfirst, filteronly and filterlast are too trivial to be a good idea. These can straightforwardly be written in terms of lazy operations which are much more generically useful that what's proposed here:

• findonly -> Iterators.findall + only
• filterfirst -> Iterators.filter + first
• filteronly -> Iterators.filter + only
• filterlast -> Iterators.reverse + Iterators.filter + first

The only issues with these I see are:

1. We don't currently have Iterators.findall (Implement lazy findall (Iterators.findall, perhaps?) #43737)
2. It's slightly annoying that the most useful versions of filter, reverse, etc. are in the Iterators namespace.

[aplavin]

findonly -> Iterators.findall + only

Aside from Iterators.findall not existing, it breaks the straightforward symmetry: first - findfirst(A) vs only - only(Iterators.findall(A)).
I can easily see people using findfirst for simplicity when they really need findonly. It's the same as was before introducing only to Julia: many used first instead.

Also, uniqueonly probably cannot be made performant without a dedicated function for it.

It's slightly annoying that the most useful versions of filter, reverse, etc. are in the Iterators namespace.

Idk, the Base ones are definitely the most useful for me. The return types and performance model of eager operations is the more straightforward one in Julia, compared to lazy Iterators.xxx. Eg, doing filter on an array (or table) gives you the same kind of object back – not some generic iterator.