Give our fsyspath an operator std::string() conversion method.

This is redundant (but harmless) on a Posix system, but it fills a missing
puzzle piece on Windows. The point of fsyspath is to be able to interchange
freely between fsyspath and std::string. Existing fsyspath could be
constructed and assigned from std::string, and we could explicitly call its
string() method to get a std::string, but an implicit fsyspath-to-string
conversion that worked on Posix would trip us up on Windows. Fix that.

indra/llcommon/fsyspath.h

@@ -69,6 +69,11 @@ class fsyspath: public std::filesystem::path
 
     // shadow base-class string() method with UTF-8 aware method
     std::string string() const { return super::u8string(); }
+    // On Posix systems, where value_type is already char, this operator
+    // std::string() method shadows the base class operator string_type()
+    // method. But on Windows, where value_type is wchar_t, the base class
+    // doesn't have operator std::string(). Provide it.
+    operator std::string() const { return string(); }
 };
 
 #endif /* ! defined(LL_FSYSPATH_H) */