Attributes to fine-tune generated bindings

Crubit recognizes custom attributes to fine-tune generated bindings.

To use a Crubit attribute, you must add the following to your Rust crate:

#![feature(register_tool)]
#![register_tool(__crubit)]

The format of the Crubit tool attributes is #[__crubit::annotate(attribute_name="attribute_value")].

cpp_name

This attribute overrides the name of the generated C++ bindings. For example, the following Rust code:

#[__crubit::annotate(cpp_name="Create")]
pub fn new() -> i32 {...}

Will generate the following C++ bindings:

std::int32_t Create(); // named `Create` instead of `new`.

Currently this attribute works on functions and structs (See b/349070421).

cpp_type

TODO(b/315382130): Rename this attribute to cpp_type and add the doc.