[librustdoc] Reform lang string token splitting

Only split doctest lang strings on `,`, ` `, and `\t`. Additionally, to
preserve backwards compatibility with pandoc-style langstrings, strip a
surrounding `{}`, and remove leading `.`s from each token.

Prior to this change, doctest lang strings were split on all
non-alphanumeric characters except `-` or `_`, which limited future
extensions to doctest lang string tokens, for example using `=` for
key-value tokens.

This is a breaking change, although it is not expected to be disruptive,
because lang strings using separators other than `,` and ` ` are not
very common

compiler/rustc_mir/src/dataflow/framework/mod.rs

@@ -10,7 +10,7 @@
 //! fixpoint solution to your dataflow problem, or implement the `ResultsVisitor` interface and use
 //! `visit_results`. The following example uses the `ResultsCursor` approach.
 //!
-//! ```ignore(cross-crate-imports)
+//! ```ignore (cross-crate-imports)
 //! use rustc_mir::dataflow::Analysis; // Makes `into_engine` available.
 //!
 //! fn do_my_analysis(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
@@ -211,7 +211,7 @@ pub trait Analysis<'tcx>: AnalysisDomain<'tcx> {
     /// default impl and the one for all `A: GenKillAnalysis` will do the right thing.
     /// Its purpose is to enable method chaining like so:
     ///
-    /// ```ignore(cross-crate-imports)
+    /// ```ignore (cross-crate-imports)
     /// let results = MyAnalysis::new(tcx, body)
     ///     .into_engine(tcx, body, def_id)
     ///     .iterate_to_fixpoint()