Handling structs without the default constructor via `Expr(:new,...` ?

In FiniteDifferences, this PR allowed custom constructors to be handled as part of to_vec: https://github.com/JuliaDiff/FiniteDifferences.jl/pull/181 here

However iiuc construct in CRC here doesn't handle this. In this case however, the constructor requirements are mentioned in the docstring.

However, tests in CRTU will construct the new type as part of the call to FiniteDifferences but then fail later as the constructor is not found in the call to construct.

Allowing this here, or otherwise, it would be easier if these behaviours were consistent in terms of custom types as part of the CTRU testing pipeline.

EDIT: MWE:

struct CustomType{T}
           x
           function CustomType(X::T) where {T}
               new{T}(X)
           end
end

# This misses the constructor:
CustomType{Int64}(5) # ERROR: MethodError: no method matching CustomType{Int64}(::Int64)

val = CustomType(5) # CustomType{Int64}(5)
diff = Tangent{typeof(val)}(x = 5) # Tangent{CustomType{Int64}}(x = 5,)

val + diff #ERROR: MethodError: no method matching CustomType{Int64}(::Int64)

# FiniteDifferences handles this in the vec to x:

to_vec(CustomType(5))[2]([6.0]) #CustomType{Int64}(6.0)

Defining CustomType{T}(x) where {T} = CustomType(x) allows this pass.

cc @mzgubic

Start with the CustomType MWE and read construct in src/differentials/composite.jl, especially the linked section. Then inspect the CRTU tests that construct the type through FiniteDifferences and compare that behavior with the custom-constructor handling described in the issue. Done means the relevant CRTU case no longer fails for this constructor pattern.