[WIP] Adapt to Julia 1.7

[dfdx]

No description provided.

[dfdx]

Currently fails in Julia 1.7 with:

cassette.jl: Error During Test at /home/az/work/Mixtape.jl/test/runtests.jl:15
  Got exception outside of a @test
  LoadError: UndefVarError: OrcJIT not defined
  Stacktrace:
    [1] _jitlink(job::GPUCompiler.CompilerJob{Mixtape.MixtapeCompilerTarget, Mixtape.MixtapeCompilerParams, GPUCompiler.FunctionSpec{typeof(Main.TestMixtape.CassetteV2.Target.foo), Tuple{Float64}}}, ::Tuple{DataType, LLVM.Module, String, String})
      @ Mixtape ~/work/Mixtape.jl/src/Mixtape.jl:504
...

It's on LLVM v4.7.0. Is it just renaming of a struct or things are more complicated?

[femtomc]

Mixtape uses GPUCompiler to link in the execution engine. This means that it has to be compat with GPUCompiler — which I think no longer exports this as an interface.

[dfdx]

Perhaps we can stick to an earlier version of GPUCompiler, but eventually we will get to incompatible set of dependencies. Any other idea how to resolve it?

[femtomc]

@dfdx I'm working on this here #19 -- looks like it will be more intensive than I thought.