Smt and SimpleSmt: Make generic over T: AsRef<Word>

[plafer]

Currently, Smt only supports values of type Word. It would be helpful to allow any type that basically wraps a Word (i.e. implements AsRef<Word>), so that we can use any methods on such type.

For example, in miden-node, we have an Smt for nullifiers. We would like to create a type struct NullifierValue(Word) which supports converting to/from BlockNum. We currently cannot implement such type, because we are forced to store Words in the Smt. See comment.