Generate C header files for safe_client_libs

Goal: Get moz_cheddar (https://github.com/mozilla/moz-cheddar) working for safe_client_libs to produce C header files.

Subtasks:
1. Find out whether C API should be in lib.rs directly or available to it via `pub use some::nested::c_module::*;` or mixed and matched as it is now.
2. If so, refactor so that where there is C API code there is nothing else, then include `pub use ...::*;` in lib.rs for those modules and see if it generates proper C header files.
3. See what can be done about converting `c_void` to `void`.

• Seems like changing `c_void` to `std::os::raw::c_void` everywhere is the only option based on https://github.com/mozilla/moz-cheddar/issues/7. We'll process the headers after generation to convert `c_char` to `char` and `c_void` to `void`. Add a task for the future to revert it back if moz-cheddar/rusty-cheddar ever fixes this.

4. Confirm that everything else in the produced header files is correct.

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