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.stderr

@@ -0,0 +1,22 @@
+error[E0277]: this type is outside the Sausage schema subset
+ --> tests/ui/subset.rs:6:12
+  |
+6 |     value: std::collections::HashMap<String,String>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SchemaType` is not implemented for `HashMap<std::string::String, std::string::String>`
+  |
+  = help: the following other types implement trait `SchemaType`:
+            ()
+            Bad
+            Vec<T>
+            bool
+            f32
+            f64
+            i16
+            i32
+          and $N others
+note: required by a bound in `supported`
+ --> tests/ui/subset.rs:3:1
+  |
+3 | #[sausage_plugin::schema]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `supported`
+  = note: this error originates in the attribute macro `sausage_plugin::schema` (in Nightly builds, run with -Z macro-backtrace for more info)