implement the ? operator

The `?` postfix operator is sugar equivalent to the try! macro, but is more amenable to chaining:
`File::open("foo")?.metadata()?.is_dir()`.

`?` is accepted on any *expression* that can return a `Result`, e.g. `x()?`, `y!()?`, `{z}?`,
`(w)?`, etc. And binds more tightly than unary operators, e.g. `!x?` is parsed as `!(x?)`.

cc #31436

src/libsyntax/ast.rs

@@ -1022,6 +1022,9 @@ pub enum ExprKind {
 
     /// No-op: used solely so we can pretty-print faithfully
     Paren(P<Expr>),
+
+    /// `expr?`
+    Try(P<Expr>),
 }
 
 /// The explicit Self type in a "qualified path". The actual