Simplify isfilled

[Whebon]

Current implementation

#HerbCore
"""
    isfilled(node::AbstractRuleNode)::Bool

Returns whether the [`AbstractRuleNode`] holds a single rule.
Holes are considered to be "filled" iff their domain size is exactly 1.
"""
isfilled(rn::RuleNode)::Bool = true
isfilled(hole::UniformHole)::Bool = (sum(hole.domain) == 1)
isfilled(hole::Hole)::Bool = (sum(hole.domain) == 1)

#HerbConstaints
function HerbCore.isfilled(hole::StateHole)::Bool
    return size(hole.domain) == 1
end

Target implementation

#HerbCore
"""
    isfilled(node::AbstractRuleNode)::Bool

Returns whether the [`AbstractRuleNode`] holds a single rule.
Holes are considered to be "filled" iff their domain size is exactly 1.
"""
isfilled(rn::RuleNode)::Bool = true
isfilled(hole::AbstractHole)::Bool = false

#HerbConstaints
function HerbCore.isfilled(hole::StateHole)::Bool
    return size(hole.domain) == 1
end

The GenericSolver automatically replaces filled holes with a rulenode, so UniformHoles and Holes will in practice always be not filled. Propagators often check this property by calling isfilled function, which currently takes $O(n)$ (for BitVectors).

We can exploit this by redefining isfilled(hole::AbstractHole)::Bool = false. However, this might cause bugs if the implementation of the GenericSolver changes.

For StateHoles, isfilled must remain the way it is, because state holes are not getting replaced by a rule node on fill. Also, for a statehole, size(hole.domain) is $O(1)$.