Reconsider self constructor

[jariji]

https://github.com/jw3126/StructHelpers.jl/blob/560b49442c361fcb8dbe65b622844566cf242074/src/StructHelpers.jl#L66

• For collection types, I normally expect a constructor to copy it rather than returning it.

julia> x = []
Any[]

julia> y = Vector(x)
Any[]

julia> x === y
false

• And for non-collection types with one field, it's not obvious that the result should even return a copy of the argument, rather than putting the argument into the field, as in a default constructor.

# Batteries
julia> mutable struct Foo 
           x
       end

julia> @batteries Foo
Foo

julia> x = Foo([])
Foo(Any[])

julia> y = Foo(x)
Foo(Any[])

# Normal

julia> struct Bar
           x
       end

julia> Bar(Bar([]))
Bar(Bar(Any[]))

Because this constructor behaves in unusual ways, I don't think it should be enabled by default. I'd lean toward just removing the option.

[jw3126]

It is a good point, people might have different expectations of a selfconstructor. I agree, that selfconstructor=false may be the better default. But I am hesitant to do that breaking change.