Make the top-level API much easier to use

[jyn514]

• Return Files instead of requiring it to be provided
• Use Program struct instead of an tuple and type alias
• Allow filename to be passed so that local includes work properly
  • This also moves filename from BinOpt to Opt

[jyn514]

For context, this is because I was reading through https://github.com/rust-lang/rust-bindgen/blob/d65100ebbaffabcc91e81ba3e88160de7cb2d5ff/src/ir/var.rs#L323 for rust-lang/rust-bindgen#1762 and noticed that the cexpr API is much nicer than what we had before. Now ours is almost as nice:

let cpp = rcc::PreProcessorBuilder::new("1 + 2").build();
// TODO: make cpp_expr() public
let expr = cpp.cpp_expr()?;