In early prototyping the template control sequence introducer was
specified as !{, but that changed to %{ along the way because it seemed
more intuitive and less likely to collide with literal strings.
However, the parser's string literal handling still had remnants of the
old syntax, causing strange quirks in parsing strings that contained
exclamation points.
Now we correctly expect %{ as the control sequence introducer, %%{ as its
escape sequence, and additionally fix a bug where previously template
sequence introduction characters at the end of a string literal would
be silently dropped due to them representing an unterminated escape
sequence.
This fixes#3.
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