feat: report incorrect padding instances

[DavePearce]

Summary

When a function is given with a padding instance that is not a valid instance of the function, then a useful error should be reported. Currently, an error is reported only during trace propagation and this is actually quite confusing.

Example

fn f(x u16) -> (y u16) {
   var c u1
   c,y = x + 1
   return
}

Then, with this trace:

{ "f" : { "x": [], "y": [] }}

We get the following error:

inconsistent instance f(0)=0 in trace (expected f(0)=1)

Approach

The approach is fairly straightforward. We just evaluate the padding instance during compilation and check the padding instance outputs match, at which point we can report a more useful error.