utils/enum_option.hh: add implicit converter to the underlying enum

Add an implicit converter of the enum_option to the underyling enum
it is holding. This is needed for using switch() on an enum_option.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

utils/enum_option.hh

@@ -87,9 +87,13 @@ class enum_option {
         return _value == that._value;
     }
 
+    // For comparison with enum values using if or switch:
     bool operator==(typename map_t::mapped_type value) const {
         return _value == value;
     }
+    operator typename map_t::mapped_type() const {
+        return _value;
+    }
 
     // For program_options parser:
     friend std::istream& operator>>(std::istream& s, enum_option<Mapper>& opt) {