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)]
```