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.

crates/rust-analyzer/src/lsp_ext.rs

@@ -121,12 +121,21 @@ pub struct RunnablesParams {
     pub position: Option<Position>,
 }
 
+// Must strictly correspond to the executable name
+#[derive(Serialize, Deserialize, Debug)]
+#[serde(rename_all = "lowercase")]
+pub enum RunnableKind {
+    Cargo,
+    Rustc,
+    Rustup,
+}
+
 #[derive(Deserialize, Serialize, Debug)]
 #[serde(rename_all = "camelCase")]
 pub struct Runnable {
     pub range: Range,
     pub label: String,
-    pub bin: String,
+    pub kind: RunnableKind,
     pub args: Vec<String>,
     pub extra_args: Vec<String>,
     pub env: FxHashMap<String, String>,