Templated constructor of String should be deleted

The following discussions from !10 (merged) should be addressed:

• @klinkovsky started a discussion: (+1 comment)

  This should be a free function, not a constructor (note that there is already convertToString which is overloaded for bool). At the very least the constructor should be explicit.

• @klinkovsky started a discussion: (+1 comment)

  The recursion should be avoided by deleting the templated constructor and using the free function convertToString explicitly instead. It can be renamed to toString or something like that to make it shorter, c.f. std::to_string.