test/serialization/general: new test.
serialization: tidy; doc tidy.

serialization/fixed_array: removed ghost code ser_match_inner (unused).
traits: added remove_ptr<T>; tidy.
string: added warnings about character arrays; tidy.

string: added defaulted cstr_tag parameter to C-string size() to
facilitate char[N] decay.
test/string/general: corrected string::size() calls to non-literals.