Add mapping to Rust Bound

[crwen]

Rust Bound has three type of value: Unbounded, Included and Excluded, but python can not express them well. So we should build a structure mapping to Rust Bound like this:

await txn.scan(Bound::Included(1), Bound::Unbounded, limit=100, projection=["age", "weight"])

[ethe]

Maybe create a Python standard enum to define Bound?

[crwen]

Maybe create a Python standard enum to define Bound?

I also have considered two ways:

• use enum, but pyo3 does not support the combination of simple enum and complex enum, so it would look like Bound::Unbounded(). Maybe None could be used to replace it
• use generic parameter and use None to express Unbounded