Allow simple enums in Input

[blueberrymuffin3]

The code for setting up the input buffers currently causes a crash when getting enums for input because it calculates the size of the input using core::mem::size_of() instead of bincode, which have different results. I wrote a new test to demonstrate the issue. Without the changes in this PR, it panics:

By updating the initial size to use bincode::serialized_size(&T::Input::default()), the problem is fixed. This does not solve for cases when the enum variants have data and are encoded to different sized, which would require a much larger change.

[caspark]

Ah, this was my fault - didn't consider this when I swapped out bytemuck for bincode serialization of inputs! Thanks for the fix.

This does not solve for cases when the enum variants have data and are encoded to different sized, which would require a much larger change.

FYI I do have support for this in my fork as a pretty self contained commit on main; hopefully I can PR it some time - if I can scrape together some free time.

[gschup]

Thanks for spotting & fixing :)