Improve output of tokens in Parse Errors

Currently, unexpected tokens in the parser are shown as the text
found, plus the internal token name, including the notorious
"unexpected '::' (T_PAAMAYIM_NEKUDOTAYIM)".

This commit replaces that with a more user-friendly format, with
two main types of token:

* Tokens which always represent the same text are shown like
  'unexpected token "::"' and 'expected "::"'
* Tokens which have variable text are given a user-friendly
  name, and show like 'unexpected identifier "foo"', and
  'expected identifer'.

A few tokens have special cases:

* unexpected token """ -> unexpected double-quote mark
* unexpected quoted string "'foo'" -> unexpected single-quoted
  string "foo"
* unexpected quoted string ""foo"" -> unexpected double-quoted
  string "foo"
* unexpected illegal character "_" -> unexpected character 0xNN
  (where _ is almost certainly a control character, and NN is the
   hexadecimal value of the byte)

The \ token has a special case in the implementation just to stop
bison making a mess of escaping it and it coming out as \\

Author: IMSoP