Prototype: transparent typedefs

[scovich]

Following https://github.com/mozilla/cbindgen/issues/967, introduce /// cbindgen:transparent-typedef annotation that causes cbindgen to replace transparent structs and typedefs with their underlying type.

This allows the following:

/// cbindgen:transparent-typedef
#[repr(transparent)]
struct Foo(NonNull<i32>);

/// cbindgen:transparent-typedef
type Function = extern "C" fn(i: i32) -> i64;

type NullableFoo = Option<Foo>;
type NullableFunction = Option<Function>;

to export as

typedef i32 *NullableFoo;
typedef int64_t (*NullableFunction)(int32_t);

instead of today's output:

template<typename T = void>
struct Option;

using Foo = int32_t*;
using Function = int64_t(*)(int32_t i);

using NullableFoo = Option<Foo>;
using NullableFunction = Option<Function>;

NOTE: This PR incorporates several other improvements that can be factored out as separate pull requests. The last commit is the actual prototype for this feature.