0.18 primitive: crate::io::Pipeline + Sink trait

Generic bounded producer/consumer pipeline. The same shape applies to
sweep, patch, and mux today via three near-duplicate implementations
(or, in patch's and mux's case, no implementation at all). 0.18
collapses them onto one primitive. See
(internal)/memory/0_18_redesign.md for full context.

Single contributor: MattJackson.

src/io/mod.rs

@@ -13,4 +13,13 @@
 mod writeback;
 mod writer;
 
+pub mod pipeline;
+
 pub(crate) use writer::Writer;
+
+// Re-exports for the 0.18 redesign. Currently flagged unused because
+// no in-tree call site has been migrated yet (sweep is still on
+// `disc/sweep_pipeline.rs`; patch and mux have no pipeline). The next
+// 0.18 slice removes this allow as it wires up the first consumer.
+#[allow(unused_imports)]
+pub use pipeline::{Apply, DEFAULT_DEPTH, Pipeline, Sink};