Fixed tag substitution.

Fixed tag substitution.

If one tag name, say 'p', is a prefix of another tag name, say 'p1',
and we try to substitute all occurences of '@p' in the given string,
we might occasionally substitute '@p1'.

I made a small re2c lexer that recognizes tag-like lexemes and
tries to match them agains rule's tags. If a tag's name matches the
recognized substring, than it is the perfect match (name cannot be
a prefix of a longer name because lexer recognizes the longest match).