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/document.rs

@@ -0,0 +1,35 @@
+//! Shared access to OpenRPC documents and their Sausage extensions.
+use serde_json::{Value, json};
+
+/// An OpenRPC document, including JSON Schema and extension fields.
+#[derive(Clone, Debug, PartialEq)]
+pub struct Document(pub Value);
+
+impl Document {
+    /// Create an empty main interface.
+    pub fn new(title: &str, version: &str) -> Self {
+        Self(
+            json!({"openrpc":"1.2.6", "info":{"title":title,"version":version},
+            "methods":[], "components":{"schemas":{}},
+            "x-sausage-endpoints":{}, "x-sausage-imports":{}}),
+        )
+    }
+
+    /// Read the method descriptions.
+    pub fn methods(&self) -> &[Value] {
+        self.0["methods"]
+            .as_array()
+            .map(Vec::as_slice)
+            .unwrap_or_default()
+    }
+
+    /// Read a named schema.
+    pub fn schema(&self, name: &str) -> Option<&Value> {
+        self.0.get("components")?.get("schemas")?.get(name)
+    }
+
+    /// Consume the document as a JSON value.
+    pub fn into_value(self) -> Value {
+        self.0
+    }
+}