rust: alloc: make `allocator` module public

rust: alloc: make `allocator` module public

Subsequent patches implement allocators such as `Kmalloc`, `Vmalloc`,
`KVmalloc`; we need them to be available outside of the kernel crate as
well.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-6-dakr@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>