Split `Type` in half: separate `RsType` and `CcType`.

This is, I think, adequately motivated by the sheer impossibility of the original design working. Consider the following compound data type:

`int*` in C++ may be `Option<&i32>` in Rust.

This cannot be expressed in the old `Type` struct because it assumed the generic layout was the same: it assumed that if the C++ type was e.g. `*<int>`, then the rust type would be shaped like `a<b>` for some type names `a` and `b`. But in this case, `*<int>` becomes `Option<&<i32>>`. And anyway, `&` takes a lifetime parameter, so this is actually `Option<&<'_, i32>>`. Just not shaped the same at all!

There are many more cases beside. Here's an incomplete list of possible type maps which are incompatible with the old layout:

* `const char*` becomes `CStr`
* function pointers AND functions -- that is, `Identity<void()>` and `Identity<void()>*` -- both can become non-pointer functions in Rust: `extern "C" fn()->()`.
* `std::span<char, std::dynamic_extent>` (2 params) becomes `&mut [u8]` (1 param)

We should change this to do the right thing now, early on, rather than later when it will be even more annoying. :)

This is, in cool news, our first major divergence from [Clif's AST proto](http://google3/third_party/clif/protos/ast.proto;l=156-171;rcl=397117954).

PiperOrigin-RevId: 399622062