Prevent API regressions

Motivation

#318 revealed a bug: two functions were missing #[no_mangle] attribute, which effectively made them absent from the API.

How can we prevent bugs arising involving deletion of a function belonging to the API?

Note: in the case of materialized views API, those functions were there, but with mangled names; so effectively, they were missing from the API.

Idea

1. Create a C source file that calls ALL functions belonging to the driver's public API.
2. Divide the full API of the original CPP Driver into two categories:
   • supported or quasi-supported.
     • by quasi-supported I mean functions that we implement, but they "do nothing" (because they are similarly no-ops in CPP Driver, or it's otherwise acceptable that we make them no-ops in CPP-Rust Driver).
   • nonsupported (not implemented).
3. For the nonsupported group, create another C source file with stub implementations of all nonsupported functions.
4. Assert that combination (test_file.c, nonsupported_stubs.c, cpp-rust-driver.a) links successfully.
   • for supported functions, we make sure that they are indeed exposed by the driver. Otherwise linker shouts at unresolved references.
   • for nonsupported functions, we make sure that they are indeed NOT exposed by the driver. Otherwise linker shouts at multiple symbols with the same name.