Add support for the "struct" module

[pschanely]

CrossHair has support for some amount of binary (de)serialization, via e.g. int.to_bytes()/int.from_bytes(), but pretty much anything going through the struct module capabilities will trigger a realization. Let's preserve symbolics there, at least in the feasible cases.

In practice, this task is mostly just re-implementing the struct module in python itself (rather than C, which is how it's implemented in CPython).