Fix on Julia 1.7

[oxinabox]

Closes #32 and #28

Vararg on 1.7 is not a type any more.
So it gets its own path through the code.
This does mean we actually generate nicer code with that.
(because it was easier to generate nicer code than workout how to generate code identical to what we had before).
But that also means we needed to put some branches in the tests where they do produce nicer code

This PR blocks fixing Nabla to work on Julia 1.7.
(though old versions of Nabla that are PreChainRules are not blocked by this. Including the version in production)

[codecov]

Codecov Report

Merging #33 (8983be6) into master (090c74b) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   97.69%   97.77%   +0.08%     
==========================================
  Files           5        5              
  Lines         217      225       +8     
==========================================
+ Hits          212      220       +8     
  Misses          5        5              

Impacted Files	Coverage Δ
src/method.jl	97.41% <100.00%> (+0.19%)	⬆️

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[iamed2]

I don't know if you want to handle this but:

In Julia 1.7, with this change:

julia> ExprTools.name_of_type(Vararg)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getproperty
   @ ./Base.jl:42 [inlined]
 [2] name_of_type(x::Core.TypeofVararg)
   @ ExprTools ~/.julia/packages/ExprTools/fES3C/src/method.jl:184
 [3] top-level scope
   @ REPL[5]:1

In Julia 1.6, with or without this change:

julia> ExprTools.name_of_type(Vararg)
:(Vararg{T, N} where {T, N})

[oxinabox]

name_of_type isn't exactly part of the public API.
But I feel that should work so I will fix that