Method operator[]

API/fleece/Mutable.hh:194–195

This enables e.g. `dict["key"_sl] = 17`

        // This enables e.g. `dict["key"_sl] = 17`
        inline keyref<MutableDict,slice> operator[] (slice key)
            {return keyref<MutableDict,slice>(*this, key);}
        inline keyref<MutableDict,slice> operator[] (const char *key)
            {return keyref<MutableDict,slice>(*this, slice(key));}
        inline keyref<MutableDict,Key&> operator[] (Key &key)