update c api to properly use the struct keyword

[ryanolson]

Rust will automatically create bindings when the C API is properly structured. Unfortunately, we were missing the struct keyword in a number of places. If you are using this header in a C++ setting, you can get away with it, but the way Rust is using LLMV to create the bindings, the C API must properly conform to the standards.

In C, you must explicitly use the struct keyword to declare a structure. In C++, you do not need to use the struct keyword after the type has been defined.

You have the option of declaring variables when the structure type is defined by placing one or more comma-separated variable names between the closing brace and the semicolon.

[ryanolson]

Error_Code doesn't need to have a struct on it.

[dyastremsky]

Looks good to me. Running through CI, then can merge if passes.