fix: Pass stdin as a ref

[wwared]

Fixes #181

CommitteeChangeIn is a large struct (250KB) and passing it by value is enough to cause a stack overflow since initializing the sphinx prover requires a lot of recursive calls while building the recursive verifier programs. Passing it as a reference to prove changes the layout somehow, enough for the stack overflow issue to disappear.

As a rule of thumb, large structs like that shouldn't be passed around by value and it's better to pass by reference to prevent unnecessary implicit copying, but we don't currently have a great way of checking which structs are too large. Besides, this case was only an issue due to the heavy stack usage when initializing the sphinx prover, otherwise 250KB is not that much. To put in perspective, the default stack size is 8MiB, so the struct uses about 3% of the available stack space. I don't know what kind of codegen is leading to the stack overflow happening.

EDIT: Actually, the default stack size seems to be 2MiB for newly spawned threads (and cargo test spawns a new thread I believe?), 8MiB is the default for the main thread only I think, which would explain why the 250KB struct is enough to hit the stack limit when passed by value, if this is happening in a new thread. I didn't try changing RUST_MIN_STACK but that would confirm the issue.

[tchataigner]

LGTM, thanks again