json: Add support for memories.

json: Add support for memories.

Previously, memories were silently discarded by the JSON backend, making
round-tripping modules with them crash.

Since there are already some users using JSON to implement custom
external passes that use memories (and infer width/size from memory
ports), let's fix this by just making JSON backend and frontend support
memories as first-class objects.

Processes are still not supported, and will now cause a hard error.

Fixes #1908.