wasm: investigate options for data sharing

Currently, each Wasm VM instance in OPA has its own representation of the data in memory.

It would be nice to have them share that, since it's read-only anyways. Memory usage shouldn't be # of VMs x data size.

To investigate:

• Is this what the "threads" proposal's shared memory covers?
  • Is our runtime capable of that?
• Are there other ways?
  • Could every data-lookup be a function call to some imported function that brokers data access? How's the performance of that?