Better support for well-known C++ types

Certain types like std::string or std::vector are quite common in C++ code, yet trying to run them through bindgen tends to be a nightmare because their implementations are actually very complicated. There are also types like string_view or llvm::StringRef that are conceptually identical to native Rust types (&str, in this case, sidestepping the issue of UTF-8/non-UTF-8) that would be useful to have represented as the native Rust type in function calls.

Rather than trying to work towards getting all the functionality necessary to build std::string (which is really a complex template that no one uses as such), it makes sense to provide a dedicated mapping mechanism specifically for the std::string type. This could perhaps be done by giving a mechanism to say "where you see type T as a function parameter in C/C++, use RustTy instead for the Rust code" and providing code to map between the C++ and Rust types.

The issue names no files, tests, or entry points. Start by locating existing C++ type handling and type-mapping code in the repository; done would require a defined mechanism for mapping well-known C++ types to native Rust types, along with agreed behavior for conversions.