Annotate Node fields as const

[tpgillam]

Make use of the new functionality in Julia 1.8 introduced here: JuliaLang/julia#43305

Should hopefully be able to annotate both members of Node, since the structure itself is only mutable to allow finalizers to run.
Two things to consider:

1. How to do this in a way that doesn't break compatibility with Julia 1.6?
2. If both fields are marked as const, will this break finalizers for the object?
3. We should test that the fields actually cannot be mutated.

TimeDag.jl/src/core.jl

Lines 14 to 25 in 2216089

	!!! warning
	Note that a `Node` is only declared mutable so that we can attach finalizers to
	instances. This is required for the [`WeakIdentityMap`](@ref) to work.
	Nodes should NEVER actually be mutated!
	Due to subgraph elimination, nodes that are equivalent should always be identical objects.
	We therefore leave `hash` & `==` defined in terms of the `objectid`.
	"""
	mutable struct Node{T}
	parents::NTuple{N,Node} where {N}
	op::NodeOp{T}
	end

1 — if no neater solution is available, should just be able to do a switch based on the VERSION >= v"1.8", so as to switch between two different struct definitions.
2 — the tests should catch this if it breaks.