C++ bindings for Rust use declarations

Crubit supports use declarations for functions and types, mapping them to equivalent using declarations in C++.

Limitations:

• The use declaration must refer to a function or type.
• The use declaration must import exactly one entity per name. For example, pub use m::x; is supported if x refers to a function, or to a type, but not if it refers to both a function and a type.

Example

Given the following Rust crate:

pub use module::function;
pub use module::Type;

Crubit will generate the following bindings:

using Type CRUBIT_INTERNAL_RUST_TYPE(":: example_crate_golden :: Type") =
    ::example_crate::Type;

  using __crubit_assert_type = ::example_crate::Type;

using ::example_crate::function;