rust: pin-init: internal: add `PhantomInvariant` and `PhantomInvariantLifetime`

Currently, the `pin_init` library has an `Invariant` type alias, and it is
instantiated using `PhantomData`. Generated code from `pin_data` on the
other hand cannot access the crate-local type alias, so it generates
`PhantomData<fn(T) -> T>` directly. This is all very inconsistent, despite
the exact same use case of ensuring invariance.

Add `PhantomInvariant` and `PhantomInvariantLifetime` and switch all users
that need to express the concept of invariance to use these. They're
polyfills of unstable types in the same names in the Rust standard library.

Link: https://patch.msgid.link/20260512-pin-init-sync-v1-3-81963130dfbd@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

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