sausage-plugin part 2: import! with typed clients (sausage_interface::client_rs), TCP endpoints with rpc.endpoint, examples

Part 1 fixes: complete stdout writes, buffered stdin, Rust type names
in schemas, enum variant docs, -32603 for serde errors in handlers,
crate docs and modules. End-to-end check of a generated consumer through
a fake core.

interface/src/lib.rs

@@ -4,3 +4,7 @@
 pub mod compat;
 pub mod document;
 pub use document::Document;
+#[cfg(feature="codegen")]
+mod client;
+#[cfg(feature="codegen")]
+pub use client::client_rs;