Add a lightweight version written with declarative macros

[loichyan]

This PR adds a lightweight version of crate_interface.

Motivation

The major reason for crate_interface_lite is the same as pin-project-lite -- using declarative macros does not require any external dependencies and could speed up the fresh builds (that is building from scratch with crate_interface requires compiling proc-macro related crates beforehand). Also, for potential library authors use this crate, crate_interface_lite would result in a tidier dependency tree of their projects.

Implementation

Due to the limitations of declarative macros, the lite one lacks supports for complex ASTs. And I have documented perceptible differences¹.

Furthermore, crate_interface_lite::def_interface uses a different approach to work with call_interface. Instead of generating a private mod to include linked extern fns², it implements the defined interface for a dummy struct, as illustrated below:

// With crate_interface_lite．．．
crate_interface_lite::def_interface!(
	trait SimpleIf {
	    fn foo() -> u32;
	}
);
impl SimpleIf for crate_interface_lite::DefaultImpl {
    fn foo() -> u32 {
        extern "Rust" {
            #[link_name = "__SimpleIf__foo"]
            fn foo() -> u32;
        }
        unsafe { foo() }
    }
}

// With crate_interface...
#[crate_interface::def_interface]
trait SimpleIf {
	fn foo() -> u32;
}
mod __SimpleIf_mod {
    use super::*;
    extern "Rust" {
        pub fn __SimpleIf_foo() -> u32;
    }
}

This should be transparent for users, since the implementation details are not exposed to them.

Future works

If you accept this PR, I would like to take the responsibility for the future maintenance of crate_interface_lite :)

Footnotes

1. Though some of them could be resolved, but it would complicate the internal macro parsing procedure. ↩

2. This is actually impossible for declarative macros unless concat_idents is stable. ↩