Publicly export `linkme` from `intertrait` and use it for the macros

Simply publicly export the linkme crate - or the needed components from it - at the root of the intertrait crate, so that macros can access their dependencies without needing people to install the linkme crate separately.

Changes

macros/src/gen_caster.rs

Old

macros/src/gen_caster.rs

#[::linkme::distributed_slice(::intertrait::CASTERS)]

New:

src/lib.rs

pub use linkme;

macros/src/gen_caster.rs

#[::intertrait::linkme::distributed_slice(::intertrait::CASTERS)]
#[linkme(crate = ::intertrait::linkme)]

先阅读 src/lib.rs 和 macros/src/gen_caster.rs，然后检查宏依赖是如何声明和引用的。按所示更新公共 crate 路径，并运行仓库的 Rust 测试或构建检查；完成的标志是宏使用 intertrait 导出的 linkme，而不需要单独的 linkme 依赖。