[Experiment] Add deferred_with

[vchuravy]

Currently the pattern to embedd Enzyme into a GPU kernel looks something like this:

function kernel(#=....=#)
    # ...
end

function dkernel(#=...=#)
    autodiff_deferred(Reverse, kernel, #=...=#)
end

Internally this would create a job for Enzyme to later process using the deferred compilation
mechanism originally built for CUDA.jl

In #582 I am reworking this to embedd the necessary information into the IR,
instead of relying on a runtime dictionary inside GPUCompiler.

For deferred compilation ala CUDA.jl we have it relativly easy. We just need to process the discovered method-instance
and link it into the output module.

For the Enzyme use-case it becomes more tricky. We can't embed a CompilerJob since that captures a lot of superflous information,
yet we do need to capture enough information to construct an approprate CompilerJob.

Taking a note from JuliaLang/julia#52964 I propose that we add something like

struct EnzymeContext
   mode
   f_activity
   rt_activity
   args_activity
   width
   #...#
end

@wsmoses essentially everything that the NonGenABI requires.

And essentially rename Enzyme.thunk to GPUCompiler.var"gpuc.deferred.with"(EnzymeContext(#=...=#), f, args...)
(actually lookup.with) since that will have the mi. An issue might be ReverseModePrimal/ReverseModeGradient since that
returns more than a single return value.

The question arises, why not just have gpuc.deferred.with and remove gpuc.deferred?
I am not sure, how the worklist integration will look like, but I for me there is a symmetry to invoke_within and invoke
in the Compiler plugins proposal. (I should probably call this gpuc.deferred.within xD )

Opening this up so that I can unload my brain for now and have it percolate a bit.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (3c80a5d) to head (0d5bcaf).
Report is 1 commits behind head on vc/rework_deferred_codegen.

Files	Patch %	Lines
src/jlgen.jl	0.00%	10 Missing ⚠️

Additional details and impacted files

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