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

[LunaticWyrm467]

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)]