improved `eltype` for `flatten` with tuple argument (#55946)
We have always had
```
julia> t = (Int16[1,2], Int32[3,4]); eltype(Iterators.flatten(t))
Any
```
With this PR, the result is `Signed` (`promote_typejoin` applied to the
element types of the tuple elements).
The same applies to `NamedTuple`:
```
julia> nt = (a = [1,2], b = (3,4)); eltype(Iterators.flatten(nt))
Any # old
Int64 # new
```
