add a public next_revision method and start on incremental test

examples/incremental/implementation.rs

@@ -0,0 +1,36 @@
+use crate::counter::Counter;
+use crate::log::Log;
+use crate::queries;
+
+#[derive(Default)]
+pub struct QueryContextImpl {
+    runtime: salsa::runtime::Runtime<QueryContextImpl>,
+    clock: Counter,
+    log: Log,
+}
+
+salsa::query_context_storage! {
+    pub struct QueryContextImplStorage for QueryContextImpl {
+        impl queries::QueryContext {
+            fn memoized2() for queries::Memoized2;
+            fn memoized1() for queries::Memoized1;
+            fn volatile() for queries::Volatile;
+        }
+    }
+}
+
+impl queries::CounterContext for QueryContextImpl {
+    fn clock(&self) -> &Counter {
+        &self.clock
+    }
+
+    fn log(&self) -> &Log {
+        &self.log
+    }
+}
+
+impl salsa::QueryContext for QueryContextImpl {
+    fn salsa_runtime(&self) -> &salsa::runtime::Runtime<QueryContextImpl> {
+        &self.runtime
+    }
+}