Add support for optional positional arguments in the dynamic DSL.

[ztangent]

Currently, the built-in modeling languages and the GFI have no support for optional trailing arguments, nor for keyword arguments. This pull request provides support for optional trailing arguments for the dynamic modeling language (but not the static modeling language), enabling the following:

@gen function foo(x, y=2, (grad)(z::Float64=3.0))
    a = @trace(normal(x+y+z, 1), :a)
    b = @trace(normal(a, 1), :b)
    return (x, y, z, x+y+z)
end

foo can now be called with optional arguments omitted:

julia> foo(5, 4)
(5, 4, 3.0, 12.0)

Traces produced by generate, simulate, update, etc., have default arguments filled in as expected:

julia>  tr, w = generate(foo, (5, 4), choicemap());
julia> get_args(tr)
(5, 4, 3.0)
julia> new_tr, _, _, _ = update(foo, (1,), (UnknownChange(),) choicemap());
julia> get_args(new_tr)
(1, 2, 3.0)

Combinators also work as expected:

julia> map_foo = Map(foo);
julia> tr, w = generate(map_foo, ([1, 2, 3], [4, 5, 6]), choicemap());
julia> get_retval(tr)[3]
(3, 6, 3.0, 12.0)

We can even do optional arguments with GenerativeFunctions as default values!

@gen function outer(inner::GenerativeFunction=foo)
    @trace(inner(1), :inner)
end

julia> foo_but_with_one_less_argument = outer;
julia> foo_but_with_one_less_argument()
(1, 2, 3.0, 6.0)

Test cases are provided in test/optional_args.jl, and the docs have been updated accordingly to characterize behavior when optional arguments are omitted.

[bzinberg]

Awesome!

Is there documentation you can update, ideally within this PR?

[ztangent]

@bzinberg Yup, I'll do that, and add some test cases to make sure combinators etc. are working with this change!

[marcoct]

@ztangent Nice job. I just had one comment.