store: move all capnp code to capnp.rs

Consolidate capnp serialization in one place:
- capnp_enum! and capnp_message! macros
- read_text/read_uuid helpers
- Type-to-capnp mappings
- from_capnp_migrate migration impls

types.rs now only has pure Rust types and helpers.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/store/view.rs

@@ -1,6 +1,7 @@
 // Read-only access abstraction for the memory store
 
 use super::types::*;
+use super::Store;
 
 // ---------------------------------------------------------------------------
 // StoreView: read-only access trait for search and graph code.