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/enum.rs

@@ -0,0 +1,8 @@
+//! Reject data-carrying enums.
+/// An invalid enum.
+#[sausage_plugin::schema]
+enum Bad {
+    /// A payload variant.
+    Value(String),
+}
+fn main() {}