Forbid newline in strings and character classes.

Forbid newline in strings and character classes.

Like C language, re2c allows all characters of current charset excapt for
newline and backslash to apper unescaped in string and class literals.

Added tests:
    - all possible characters in strings and classes (omitting
      newline, escaping backslash and quotes/closing bracket)
      with different encodings (re2c up to 0.14.x would segfault
      on this test with '-u' and '-w')
    - string with newline
    - string with unescaped backslash followed by newline (so that
      it won't stick to next character)