More maintainable config

Rather than eagerly converting JSON, we losslessly keep it as is, and
change the shape of user-submitted data at the last moment.

This also allows us to remove a bunch of wrong Defaults

crates/rust-analyzer/src/to_proto.rs

@@ -818,7 +818,7 @@ pub(crate) fn runnable(
     file_id: FileId,
     runnable: Runnable,
 ) -> Result<lsp_ext::Runnable> {
-    let config = &snap.config.runnables;
+    let config = snap.config.runnables();
     let spec = CargoTargetSpec::for_file(snap, file_id)?;
     let workspace_root = spec.as_ref().map(|it| it.workspace_root.clone());
     let target = spec.as_ref().map(|s| s.target.clone());
@@ -833,9 +833,9 @@ pub(crate) fn runnable(
         kind: lsp_ext::RunnableKind::Cargo,
         args: lsp_ext::CargoRunnable {
             workspace_root: workspace_root.map(|it| it.into()),
-            override_cargo: config.override_cargo.clone(),
+            override_cargo: config.override_cargo,
             cargo_args,
-            cargo_extra_args: config.cargo_extra_args.clone(),
+            cargo_extra_args: config.cargo_extra_args,
             executable_args,
             expect_test: None,
         },