Use word_opt instead of tag_opt

[moratorium08]

Changes Made

I changed tag_opt to word_opt to handle identifiers correctly.

Bugs Fixed

Previously, there was a bug that produced an error like the following:

(declare-fun andx (Int) Bool)
(assert (andx 1))
(error "
  unknown identifier x at [2:13]:
    |
  2 | (assert (andx 1))
    |             ^
")

This occurred because HoIce's parser behaved as follows:

if self.tag_opt("and") {
                stack.push(Frame::And(vec![]));
                continue 'go_down;

tag_opt only checks if the first three characters match "and", but it doesn't verify if the next character is an alphabetic character or something that can be part of an identifier.

The correct functionality is provided by word_opt, so I updated some uses of tag_opt to word_opt.