Fix type-instability of ∘ and Some when wrapping types

[tkf]

It looks fixing type instability of closure is hard (#35970). So, can we add a workaround for cases like Float64 ∘ first?

fix #34849

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I also added a similar fix for Some

Before

julia> typeof(Float64 ∘ first)
Base.var"#62#63"{DataType,typeof(first)}

julia> typeof(Some(Int))
Some{DataType}

After

julia> typeof(Float64 ∘ first)
Base.ComposedFunction{Type{Float64},typeof(first)}

julia> typeof(Some(Int))
Some{Type{Int64}}

[JeffBezanson]

There is already a fallback for this.

[tkf]

Without this, text/plain output is rather ugly:

julia> uppercase∘first
(::Base.ComposedFunction{typeof(uppercase),typeof(first)}) (generic function with 1 method)

I think this is due to show(io::IO, ::MIME"text/plain", f::Function).

But defining show here was incorrect because MIME is not defined here. I moved it to show.jl where show(io::IO, ::MIME"text/plain", f::Function) is defined.

With this definition, I get

julia> uppercase∘first
uppercase ∘ first

[JeffBezanson]

Oh, I see, I was missing that it's a subtype of Function.

[JeffBezanson]

Yes I think this is a reasonable approach for now. For example we already do this for Generators.

[nalimilan]

This is clearly and improvement for composed functions, but is it so clear for Some? I can't decide whether it's better to have Some(Int) be a Some{Type{Int}} or a Some{DataType}. For example [Int] isa Vector{DataType}... Do we have concrete examples that could justify one choice or the other?

[tkf]

It is somewhat unfortunate that typeof([Int]) === Vector{DataType} because, IIUC, it is highly discouraged to dispatch on the representation of the type. People still can use [Int] isa Vector{<:Type} but a lot of users may not notice if they see Vector{DataType} in the REPL.

Do we have concrete examples that could justify one choice or the other?

Is type-stability not enough to justify the change?