string with fixed but compile-time configurable capacity

[FerdinandSpitzschnueffler]

Brief feature description

For a fine grained control of the size of strings in different parts of the project, a string with a fixed but compile-time configurable capacity is required. Furthermore, throwing exceptions or using heap is forbidden.

Detailed information

This can be achieved by integrating the new cxx::string which uses a template parameter for its capacity. It also provides a std::string compatible comparison which compares past the first '\0'.

[mossmaurice]

Part of #59 and #70