Add a Compare function

Since a lot of the tpmdirect structs have private members, we should create a convenient way to compare two values without hassling with adding enough types into `cmpopts.IgnoreUnexportedFields`.

Start by locating the tpmdirect structs and reviewing how cmpopts.IgnoreUnexportedFields is currently used when comparing them. Define the intended Compare API and its behavior for these private members, then add coverage demonstrating comparisons between equivalent and differing values.

Written by the indexing model from the issue text.