String: Add str_if_stable() as a const alternative to str()

The `str()` method must be non-const because it may need to internally
mutate the representation of the string in order to have an owned
`std::string` instance holding the exact string (not a superstring).
This is inconvenient in contexts where we can ensure that no mutation
is needed to get a `std::string const&`.

Add a `str_if_stable() const` method that returns `std::string const*`
so we can return `nullptr` if if mutation would be necessary to get a
`std::string const&`.  Add supporting `is_stable() const` and
`stabilize()` methods to check and enforce stable availability of
`std::string const&`.  These can be used to create `String const`
instances from which we can still get a `std::string const&` via
`*str_if_stable()` by maintaining the stability invariant at runtime.

Source/cmString.hxx

@@ -348,6 +348,20 @@ public:
 
   char back() const noexcept { return view_.back(); }
 
+  /** Return true if this instance is stable and otherwise false.
+      An instance is stable if it is in the 'null' state or if it is
+      an 'owned' state not produced by substring operations, or
+      after a call to 'stabilize()' or 'str()'.  */
+  bool is_stable() const;
+
+  /** If 'is_stable()' does not return true, mutate so it does.  */
+  void stabilize();
+
+  /** Get a pointer to a normal std::string if 'is_stable()' returns
+      true and otherwise nullptr.  The pointer is valid until this
+      instance is mutated or destroyed.  */
+  std::string const* str_if_stable() const;
+
   /** Get a refernce to a normal std::string.  The reference
       is valid until this instance is mutated or destroyed.  */
   std::string const& str();