- The `cmDefinitions::Def` struct is simplified by removing an unused
  constructor and the `std_string` type definition.  The std::string value
  becomes a member variable instead of being derived from.

- The `cmDefinitions::Unset` method is added that unsets a definition.
  It has the same functionality as calling `cmDefinitions::Set` with a
  `nullptr` value argument,

- The `cmDefinitions::Set` method gets an overloaded version that takes a
  `cm::string_view` as value argument.

- The originl `cmDefinitions::Set` method with `const char*` argument is
  refactored to either call the `cm::string_view` based `cmDefinitions::Set`
  overload or `cmDefinitions::Unset`, depending on whether the value
  `const char*` is a nullptr (`Unset`) or not (`Set`).