static_show: Use reinterpret for types with non-default constructors

[topolarity]

The idea here is that we can reasonably expect that types with default inner constructors can be constructed as Foo(field1, field2, field3) in most cases. That's because, if an outer constructor "covers" the full-length default constructor, then it can only construct the object it needs via @invoke, reinterpret, Expr(:new, ...) or similar workarounds (the full-length default constructor is not available via dispatch and neither is new)

This gives us a compact output for the majority of isbits types:

julia> struct Foo
           x::Int
           y::Int
           z::Int
       end
julia> ccall(:jl_, Cvoid, (Any,), Foo(1,2,3))
Foo(1, 2, 3)

but also evaluates to the correct value when inner constructors are present that do strange things:

julia> struct Bar
           x::Int
           y::Int
           z::Int
           Bar(z, y, x) = new(x, y, z)
       end
julia> ccall(:jl_, Cvoid, (Any,), Bar(1,2,3))
Base.reinterpret(Bar, (1, 2, 3))

Mostly a proof-of-concept to illustrate this approach. Lots of tests still needed (and hasdefaultctors may need to be tracked very differently)