setup CI and tidy to use typos for spellchecking and fix few typos

compiler/rustc_ast/src/ast.rs

@@ -1344,7 +1344,7 @@ impl Expr {
         }
     }
 
-    /// Returns an expression with (when possible) *one* outter brace removed
+    /// Returns an expression with (when possible) *one* outer brace removed
     pub fn maybe_unwrap_block(&self) -> &Expr {
         if let ExprKind::Block(block, None) = &self.kind
             && let [stmt] = block.stmts.as_slice()

compiler/rustc_ast/src/visit.rs

@@ -142,7 +142,7 @@ macro_rules! common_visitor_and_walkers {
             )?
 
             // Methods in this trait have one of three forms, with the last two forms
-            // only occuring on `MutVisitor`:
+            // only occurring on `MutVisitor`:
             //
             //   fn visit_t(&mut self, t: &mut T);                      // common
             //   fn flat_map_t(&mut self, t: T) -> SmallVec<[T; 1]>;    // rare