Eras mode

[oxinabox]

Closes #252

• changes to the internals
• hooking up for forward_demand
• tests

[oxinabox]

I have started adding tests but right now they show no change unnested,
and once we nest type inference is failing with or without eras mode being turned on.
So that needs to be fixed.
I suspect it is because I am using a if statement rather than a seperate method, in shuffle_up

[oxinabox]

A question for the reviewer.
Is this a breaking change?
It doesn't break the public API. and technically doesn't break calling the private ∂☆{1}()(...) API
So I guess not?

[oxinabox]

idk why CI isn't working on 1.11, it is complaining it can't compile Cthulhu.

On 1.10 it is because the code for running inference manually (which we do in the tests) is wrong, and idk how to do it right. (can probably work it out though).

[oxinabox]

• 1.11-nightly is broken due to Precompilation error on julia master due to IOError: stream is closed JuliaDebug/Cthulhu.jl#541 which is fixed on release-1.11, so we are waiting for a new julia release to be cut there
• nightly is broken for reasons related to this PR I don't yet undestand. I can't reproduce locally not even with exact same version of nightly.

[oxinabox]

fixed everything, this is now just waiting for review

[Keno]

Seems good to me.