Change Runnable.bin -> Runnable.kind

As per matklad, we now pass the responsibility for finding the binary to the frontend.
Also, added caching for finding the binary path to reduce
the amount of filesystem interactions.

editors/code/src/lsp_ext.ts

@@ -45,10 +45,13 @@ export interface RunnablesParams {
     textDocument: lc.TextDocumentIdentifier;
     position: lc.Position | null;
 }
+
+export type RunnableKind = "cargo" | "rustc" | "rustup";
+
 export interface Runnable {
     range: lc.Range;
     label: string;
-    bin: string;
+    kind: RunnableKind;
     args: string[];
     extraArgs: string[];
     env: { [key: string]: string };