Equality of Composites with different backings causes stackoverflow

[oxinabox]

see:

julia> tup = (1.0, 2.0)
(1.0, 2.0)

julia> c_tup1 = Composite{typeof(tup)}(1.0, 2.0)
Composite{Tuple{Float64,Float64}}(1.0, 2.0)

julia> c_tup2 = Composite{typeof(tup)}(1.0, @thunk(2*1.0))
Composite{Tuple{Float64,Float64}}(1.0, Thunk(var"#89#90"()))

julia> c_tup1 == c_tup2
ERROR: StackOverflowError:
Stacktrace:
 [1] ==(::Composite{Tuple{Float64,Float64},Tuple{Float64,Float64}}, ::Composite{Tuple{Float64,Float64},Tuple{Float64,Thunk{var"#89#90"}}}) at /Users/oxinabox/JuliaEnvs/ChainRulesWorld/ChainRulesCore.jl/src/differentials/composite.jl:52 (repeats 79982 times)

I think this should be a fairly easy one to debug and fix so marking it a [good first issue]