sausage-plugin part 1: run, #[rpc], #[schema], rpc.discover, calls, locks, Ctx, hooks

Checkpoint. Known issues, fixed in part 2: short writes on stdout,
byte-wise stdin reads, Result suffix on all result schemas, lost enum
variant docs, serde errors as invalid params, missing item docs.

interface/src/lib.rs

@@ -2,3 +2,5 @@
 //! (`compat`), and the generator of client code for `import!`.
 
 pub mod compat;
+pub mod document;
+pub use document::Document;