API for isolating native extensions into their own memdoms

[masomel]

Addresses #10

• [x] Create native library compartment
• [x] Allocate native library and its dependencies into its own memory domain
• [X] PyCFunction-to-thread run function args converter
• [X] Python-specific thread run function
• [ ] grant/revoke main thread access to domains?
• [ ] tests

[masomel]

Closing since we have put #17 on hold.