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.

sdk/tests/ui/signature.rs

@@ -0,0 +1,8 @@
+//! Reject invalid receiver signatures.
+struct Bad;
+#[sausage_plugin::rpc]
+impl Bad {
+    /// An invalid receiver.
+    fn bad(&mut self) -> sausage_plugin::Result<()> {Ok(())}
+}
+fn main() {}