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 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".

As a special-case, quoted strings are not quoted an extra time,
so show like "unexpected quoted string 'foo'" rather than ''foo''.

Author: IMSoP