use Base.Experimental macros to reduce load time

[mcabbott]

Aims to reduce startup time, xref #1126 . Needs more benchmarking, to see if this slows down other things.

Here's one way to get awful results:

julia> @btime gradient((x,y) -> sum(sqrt.(x./2 .+ y) .- y' .+ 10 .* x.^2), $(rand(100)), $(rand(100)));
  min 6.625 μs, mean 19.931 μs (22 allocations, 167.00 KiB)   # 0.6.33
  min 43.125 μs, mean 56.539 μs (24 allocations, 167.02 KiB)  # PR
  min 131.416 μs, mean 142.481 μs (27 allocations, 167.06 KiB)  # julia -O1, and Zygote 0.6.33

I suspect for this to be useful, you would have to partition the package into sub-modules, only some of which are -O1. It seems quite messy to do that.

[ToucheSir]

Would isolating just the compiler internals work? i.e. still optimize the code that Zygote generates, but don't optimize the code that does that generation.

[mcabbott]

Yes, that's my hope with the submodule idea -- either put all the internals in Zygote.Slow, or else leave them in Zygote and put the run-time rules in Zygote.Fast. But the code isn't organised to make this division very easy, there are dozens of weird functions which seem to cross the border.