Allow Zygote v0.7

[mcabbott]

Aims to solve errors like this, because the new Zygote allows thunks:

ERROR: MethodError: no method matching (::var"#170#∇ℳ𝒶𝓀ℯ#46"{…})(::ChainRulesCore.Thunk{…}, ::Array{…}, ::Array{…}, ::Array{…})
The function `#170#∇ℳ𝒶𝓀ℯ` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  (::var"#170#∇ℳ𝒶𝓀ℯ#46")(::AbstractArray{𝒯}, ::Any, ::Any, ::Any) where 𝒯
   @ Main ~/.julia/packages/Tullio/vChiX/src/macro.jl:1295

Stacktrace:
  [1] (::TullioChainRulesCoreExt.var"#tullio_back#2"{…})(Δ::ChainRulesCore.Thunk{…})
    @ TullioChainRulesCoreExt ~/.julia/packages/Tullio/vChiX/ext/TullioChainRulesCoreExt.jl:9
  [2] (::Zygote.ZBack{TullioChainRulesCoreExt.var"#tullio_back#2"{…}})(dy::ChainRulesCore.Thunk{Zygote.var"#167#168"{…}})
    @ Zygote ~/.julia/packages/Zygote/ZtfX6/src/compiler/chainrules.jl:221
...

Tests of this package are a bit of a disaster but I don't have time to dig into them.