Consider adding a C API to set a context's message consumer

There is no C API to set the message consumer for an spv_context.

The C API would have to be a pointer-to-function as the consumer.

On the other hand, I think I'd like a *simpler* message consumer that just consumes error strings. Should the C-based consumer be that simpler thing? Or should we bridge the gap with a easily-accessible message-to-string adapter?

Start by examining the existing spv_context interface and message-consumer APIs mentioned in the issue. Resolve whether the C API should accept a function pointer for full messages or expose an error-string consumer, then define the API and its completion criteria after that design decision.

Written by the indexing model from the issue text.