Can't pass concrete struct to main

[iAmMichaelConnor]

Please see this PR which exemplifies the issue zkpassport/noir_rsa#2. See the section "A note to the Noir team".

Edit: here's a dedicated repo focusing on the problem: https://github.com/iAmMichaelConnor/example_sized_types_entrypoint_bug

I'll see if I can come up with a more minimal example.