Are `BasicBlockOwned` and related structs a performance bottleneck for Python users?

[asaites]

The conversions to/from these Owned types and their non-Owned counterparts involves a lot of cloning, and they're the types exposed by the Python bindings. At present, this is just a code-smell, and might not be important enough to care about, so we should get some benchmark data, and if it is an issue, determine if we can combine their relevant methods or otherwise avoid the costly conversions.