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/serde.stderr

@@ -0,0 +1,5 @@
+error: serde attribute is outside the schema subset
+ --> tests/ui/serde.rs:6:13
+  |
+6 |     #[serde(flatten)]
+  |             ^^^^^^^