hevm: Toggle stack representation

Hevm used to display stack items with the item index, then the hex representation of the value with the decimal representation (or something more informative, like `keccak("setUp()")`) below. The hex representation has been replaced with the expression used to generate the value, if any.

The expression can be very useful, especially for symbolic tests. It's also just cool. But, it's often nice to just see the hex representation without having to convert the decimal representation in another window. For example, if the stack item is a memory offset or if we are just debugging a transaction and only need the concrete values.

It would be great to have a key to toggle what is displayed as the top element for each stack item between the symbolic and concrete representations. This may cause some issues when only the symbolic representation is available, so maybe toggling the bottom element between decimal / hex encodings would be simpler.

Alternatively, I personally want to use the hex representation more often than the decimal, so just replacing the bottom element with hex might be palatable for most users.