[Bridges.Variable] change ParameterToEqualToBridge to <: SetMapBridge

[odow]

@blegat it doesn't look like a SetMapBridge{T,<:MOI.AbstractScalarSet} bridge was ever implemented or tested?

[odow]

Okay, one is implemented here in test:

MathOptInterface.jl/test/Bridges/identity_bridge.jl

Lines 12 to 32 in 3bbca2a

	const F{T} = MOI.ScalarAffineFunction{T}
	const S{T} = MOI.EqualTo{T}
	struct VariableBridge{T} <: MOI.Bridges.Variable.SetMapBridge{T,S{T},S{T}}
	variable::MOI.VariableIndex
	constraint::MOI.ConstraintIndex{MOI.VariableIndex,S{T}}
	end
	struct ConstraintBridge{T} <:
	MOI.Bridges.Constraint.SetMapBridge{T,S{T},S{T},F{T},F{T}}
	constraint::MOI.ConstraintIndex{F{T},S{T}}
	end
	const IdentityBridge{T} = Union{VariableBridge{T},ConstraintBridge{T}}
	MOI.Bridges.map_set(::Type{<:IdentityBridge}, set::S) = set
	MOI.Bridges.inverse_map_set(::Type{<:IdentityBridge}, set::S) = set
	MOI.Bridges.map_function(::Type{<:IdentityBridge}, func) = func
	MOI.Bridges.inverse_map_function(::Type{<:IdentityBridge}, func) = func
	MOI.Bridges.adjoint_map_function(::Type{<:IdentityBridge}, func) = func
	MOI.Bridges.inverse_adjoint_map_function(::Type{<:IdentityBridge}, func) = func

but it doesn't get properly tested, so we don't hit the relevant methods:

MathOptInterface.jl/test/Bridges/bridge_optimizer.jl

Lines 713 to 729 in 3bbca2a

	include("identity_bridge.jl")
	function test_recursive_model_variable(::Type{T} = Int) where {T}
	model = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{T}())
	BT = IdentityBridges.VariableBridge{T}
	b = MOI.Bridges.Variable.SingleBridgeOptimizer{BT}(model)
	x, cx = MOI.add_constrained_variable(b, MOI.EqualTo(one(T)))
	@test MOI.Bridges.is_bridged(b, x)
	@test MOI.Bridges.is_bridged(b, cx)
	@test MOI.get(b, MOI.ConstraintFunction(), cx) == x
	@test MOI.get(b, MOI.ConstraintSet(), cx) == MOI.EqualTo(one(T))
	MOI.set(b, MOI.ConstraintSet(), cx, MOI.EqualTo(zero(T)))
	@test MOI.get(b, MOI.ConstraintSet(), cx) == MOI.EqualTo(zero(T))
	@test MOI.is_valid(b, x)
	MOI.delete(b, x)
	@test !MOI.is_valid(b, x)
	end

There is, for example, no check for NumberOfVariables or ListOfVariableIndices.

[blegat]

It's a bit difficult to review if the methods are moved around in the same PR so it's unclear what changed but it looks correct

[blegat]

This is much better