`muxHandle` is wrong

[langston-barrett]

Crucible doesn't really support muxing function handles, but it does have this muxHandle operation:

https://github.com/GaloisInc/crucible/blob/3ed848aee186f63a543ccebf334e3b62b38400e7/crucible/src/Lang/Crucible/Simulator/RegValue.hs#L269-L277

This is plainly wrong, as it just returns the first FnVal in the case that the predicate isn't statically known. It should probably panic instead. This code is also reachable via this CanMux instance:

https://github.com/GaloisInc/crucible/blob/3ed848aee186f63a543ccebf334e3b62b38400e7/crucible/src/Lang/Crucible/Simulator/RegValue.hs#L279-L282

Both of these are exported.

I'd advocate for:

• Making a tryMuxHandle function that returns a Maybe
• Changing muxHandle to call tryMuxHandle and panic on Nothing
• Removing the CanMux instance, because it is likely to lead to surprises

However, all of these except the first are breaking changes.