`@vnt` helper macro

[penelopeysm]

Eventually, we want to use VNT for everything in the world, including providing conditioning values.

However, with the current options (i.e. Dict), it's not possible to provide a template for array values;

model | Dict(@varname(x[1]) => 1.0, @varname(x[2]) => 1.0)

when converting this Dict to a VNT, there is no template for x, so the behaviour may either be wrong, or it may issue spurious warnings.

Of course, a user can fix this by calling templated_setindex!! themselves, although that is slightly inconvenient. It would be nice to have a macro that looks something like this:

x = zeros(10, 10)
y = (; a=zeros(5))
vnt = @vnt begin
    @template x, y
    x[1] = 10.0
    x[2, 5] = 20.0
    y.a[1] = 30.0
    z[2] = 40.0  # z has no template, so GrowableArray
end

This would desugar to a series of consecutive calls to templated_setindex!! or setindex!! as appropriate.

This would also be immensely helpful for writing tests, because it is a genuine pain to set up a templated VNT with lots of different things.

[penelopeysm]

This technically doesn't even have to live in DynamicPPL, so if people really don't like having a macro, I'll just make it in a separate package myself.

[penelopeysm]

Closed by #1236

[penelopeysm]

Modified it such that lhs := rhs is the statement that sets lhs inside the VNT. The reason is because I want to reserve the lhs = rhs syntax for doing 'normal Julia' intermediate computations, e.g. something like this

vnt = @vnt begin
    # These are just normal Julia things
    y = fill(missing, 6)
    y[1] = 1.0
    y[2] = 2.0
    # This actually sets the value of `y` in the VNT
    y := y
end

Right now it doesn't work, but I don't want to try to make it work in the future and then realise that I have to change the syntax.