Convenience functions for `cxx::string`

[FerdinandSpitzschnueffler]

Brief feature description

This is an issue to collect convenience functions for the cxx::string which would be nice to have but are not urgently needed.

Detailed information

• add c'tor that constructs the iox::string with count copies of a character

  iox::string<10> str(TruncateToCapacity, count, 'M');    // count as runtime parameter
  iox::string<10> str(count, 'M');    // count as compile time constant
  

• remove characters from the iox::string

  iox::string<10> str("someString");
  str.erase(5, 2);    // str == someSing
  str.erase(7);    // str == someSin
  

[elBoberido]

Nice. But is TruncateToCapacity necessary? I guess yes when count is a runtime parameter but can we also have one with a compile time constant and then just a iox::cxx::string<10> a(5, 'M'); with a compile time error on iox::cxx::string<10> a(15, 'M'); ?