Create single module for shared functionality

[bendk]

We should look into creating a single module on the foreign side that holds all shared functionality: RustBuffer, Future handling code, etc. There would be a couple advantages to this:

• Smaller bindings
• Simplifies compatibility issues with external types. With our current system, we create a separate RustBuffer class for each module, which makes using an external type weird since it each module uses a different RustBuffer.
• Simplifies global callback registration, for example the foreign executor callback that we used to have.