Add a clang-tidy check in the regular CI run

[randombit]

No description provided.

[reneme]

In #3619 you converted some struct to class. Was that also due to a clang-tidy rule by any chance? If yes: maybe it's worth adding that here as well. Mea culpa. 👼

[randombit]

I don't think there is a clang-tidy rule about this (though if there is I'd like to enable it).

The logic here is: plain struct, if ever used, should just be a C-style struct. If it has a constructor or associated logical functions, it should be a class. Also, unless there truly is no logical consistency requirements (which is rare!), it should be a class, and any mutations performed through setters. This allows easier understanding of how/when something is modified. Eg if the member variable is just public, then it's very easy to capture it by mutable reference in a way that is not so obvious:

struct S { int foo; int something; }

int bar(S& s) { 
  s.something = 9;
  return baz(s.foo);
}

/// in some other file far away ...
int baz(int& x) { x = 5; return 9; }

whereas if there are getters and setters than mutations are highly visible/grepable. Also in many cases no setter is required (for example the identity field in Client_PSK) in which case the absence of a setter at all makes it clear this field is never modified.

[coveralls]

coverage: 91.74% (+0.02%) from 91.725% when pulling 65d3589 on jack/fast-clang-tidy-in-ci into c6a09af on master.