Add an immutable version of CompGraph

[DrChainsaw]

Basically an alternative to #102 which is a bit less attractive since it requires creating a completely new structure.

Current version does not have tests yet but is tested with a couple of random networks from NaiveGAflux. Unfortunately (or perhaps fortunately) the performance with Zygote seems to be way worse than #102.

This is a bit surprising however since the generated function completely unrolls the graph, so maybe it is worth revisiting. For example:

## gg is a CompGraph generated by NaiveGAflux
gs = ImmutableCompGraph(gg)
compgraphexpr(typeof(gs), :g) = quote
    v15 = g.outputs
    (v14,) = v15.inputs
    (v13,) = v14.inputs
    (v12,) = v13.inputs
    (v11,) = v12.inputs
    (v7, v10) = v11.inputs
    (v6,) = v7.inputs
    (v5,) = v6.inputs
    (v4,) = v5.inputs
    (v0, v3) = v4.inputs
    (v2,) = v3.inputs
    (v1,) = v2.inputs
    v1_out = v1(v0_in)
    v2_out = v2(v1_out)
    v3_out = v3(v2_out)
    v4_out = v4(v0_in, v3_out)
    v5_out = v5(v4_out)
    v6_out = v6(v5_out)
    v7_out = v7(v6_out)
    (v9,) = v10.inputs
    (v8,) = v9.inputs
    v8_out = v8(v5_out)
    v9_out = v9(v8_out)
    v10_out = v10(v9_out)
    v11_out = v11(v7_out, v10_out)
    v12_out = v12(v11_out)
    v13_out = v13(v12_out)
    v14_out = v14(v13_out)
    v15_out = v15(v14_out)
end

Main "trick" to make it possible to do the above in type domain is to add an isbits identifier in the vertex type. Current version uses an Int which is just a running number of how many ImmutableCompVertex has been created (per call to ImmutableCompGraph).

Other options tried but which didn't give sufficient performance was to index the inputs instead of destructuring, e.g v7 = v11.inputs[1] and v10 = v11.inputs[2] instead of (v7, v10) = v11.inputs.

Also tried:

• RuntimeGeneratedFunctions
• Storing a tuple of vertices in topological order and use that to extract the v0, v1 etc variables. A bit worse performance iirc.
• Adding a function f(g, v0_in) expression on top of the above and just evaling it on the top level. The fact that this doesn't improve performance is probably an indication that a different type of expression is needed. Perhaps Zygote doesn't like to compile long functions?
• All of the above except the expression is changed to work on CompGraph instead of ImmutableCompGraph. This was significantly worse for all versions.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 64 lines in your changes missing coverage. Please review.

Project coverage is 89.58%. Comparing base (df65628) to head (0a2b072).
Report is 46 commits behind head on master.

Files with missing lines	Patch %	Lines
src/staticgraph.jl	0.00%	64 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   94.16%   89.58%   -4.59%     
==========================================
  Files          14       15       +1     
  Lines        1251     1315      +64     
==========================================
  Hits         1178     1178              
- Misses         73      137      +64     

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