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/subset.rs

@@ -0,0 +1,8 @@
+//! Reject unsupported field types.
+/// An invalid schema.
+#[sausage_plugin::schema]
+struct Bad {
+    /// An unsupported dictionary.
+    value: std::collections::HashMap<String,String>,
+}
+fn main() {}