NamedTuple constructor support for FieldArray subtypes

[cadojo]

This PR was originally posted to StaticArraysCore.jl, but was moved to here because StaticArraysCore lacks the method definitions necessary to make this NamedTuple functionality work.

I have found NamedTuple to be a performant way to initialize FieldArray subtypes. See, for example, the following code in AstrodynamicalModels.jl. I think it may also be useful sometimes to go in reverse: convert a FieldArray subtype back into a NamedTuple. Do you agree this may be useful, and without undesirable side effects? I got this idea from SciML's Base.NamedTuple method for LabelledArrays.jl. Thanks for reading!

"""
A mutable vector, with labels, for 6DOF Cartesian states.
"""
Base.@kwdef mutable struct CartesianState{F} <: FieldVector{6,F}
    x::F = 0.0
    y::F = 0.0
    z::F = 0.0
    ẋ::F = 0.0
    ẏ::F = 0.0
    ż::F = 0.0

    CartesianState{F}(::UndefInitializer) where {F} = new{F}()
    CartesianState(::UndefInitializer) = CartesianState{Float64}(undef)

    CartesianState{F}(x, y, z, ẋ, ẏ, ż) where {F} = new{F}(x, y, z, ẋ, ẏ, ż)
    CartesianState(x, y, z, ẋ, ẏ, ż) = new{promote_type(typeof(x), typeof(y), typeof(z), typeof(ẋ), typeof(ẏ), typeof(ż))}(x, y, z, ẋ, ẏ, ż)
    CartesianState{F}(state::NamedTuple) where {F} =
        let
            (; x, y, z, ẋ, ẏ, ż) = merge((; x=zero(F), y=zero(F), z=zero(F), ẋ=zero(F), ẏ=zero(F), ż=zero(F)), state)
            CartesianState{F}(x, y, z, ẋ, ẏ, ż)
        end
    CartesianState(state::NamedTuple) = CartesianState{Float64}(state)
end

[mateuszbaran]

I think this is useful.

[mateuszbaran]

Could you just add a test for convert and bump patch version?

[cadojo]

Is this okay as-is, despite the julia-nightly failure? I am assuming that is a Julia development branch quirk and not a problem with this branch.

[mateuszbaran]

There is no julia-nighty failure on the latest commit but even if there was, it would be fine as long as the failure is not related to changes in the PR.