Disallow non-printable characters in Strings

[vaivaswatha]

While #637 disallowed most non-printable characters, the parser and the function validate_string_literal allows a few whitespace characters.

Since there seems to be no use of allowing non-printable characters (< 32 or > 126), we can remove them (it is however a backwards incompatible change as we, today, allow few whitepspace characters to be escaped).

This involves the following changes.

1. In ScillaLexer.mli, we have this:

and read_string buf =
  parse
  | '"'       { STRING (Buffer.contents buf) }
  | '\\' '\\' { Buffer.add_char buf '\\'; read_string buf lexbuf }
  | '\\' '/'  { Buffer.add_char buf '/'; read_string buf lexbuf }
  | '\\' 'b'  { Buffer.add_char buf '\b'; read_string buf lexbuf }
  | '\\' 'f'  { Buffer.add_char buf '\012'; read_string buf lexbuf }
  | '\\' 'n'  { Buffer.add_char buf '\n'; read_string buf lexbuf }
  | '\\' 'r'  { Buffer.add_char buf '\r'; read_string buf lexbuf }
  | '\\' 't'  { Buffer.add_char buf '\t'; read_string buf lexbuf }
  | '\\' '"'  { Buffer.add_char buf '"'; read_string buf lexbuf }
  | [^ '"' '\\']+
    { Buffer.add_string buf (Lexing.lexeme lexbuf);
      read_string buf lexbuf
    }
  | _ { raise (Error ("Illegal string character: " ^ Lexing.lexeme lexbuf)) }
  | eof { raise (Error ("String is not terminated")) }

We need to ignore all escapes except for \\ and \" (as they are both needed to allow ", a printable character, in strings).

2. Modify validate_string_literal: remove exceptions made there for whitespaces.
3. Update test case tests/typecheck/good/str-nonprint-char-1.scilexp