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

### New:
src/lib.rs
```rust
pub use linkme;
```

macros/src/gen_caster.rs
```rust
#[::intertrait::linkme::distributed_slice(::intertrait::CASTERS)]
#[linkme(crate = ::intertrait::linkme)]
```

Read src/lib.rs and macros/src/gen_caster.rs first, then inspect how the macro dependency is declared and referenced. Update the public crate path as shown and run the repository's Rust tests or build checks; done means the macros use intertrait's exported linkme without requiring a separate linkme dependency.

Written by the indexing model from the issue text.