This requires some extra state-keeping because we allow templates to be
nested inside templates. This takes us outside of the world of regular
languages, but we accept that here because it makes things easier to
deal with down the line in the parser.
The methodology is to keep track of how many braces are open at a given
time and then, when a nested template interpolation begins, record the
current brace level. Then, when a closing brace is encountered, if its
nesting level is at the top of the stack then we pop off the stack and
return to "main" parsing mode.
Ragel's existing idea of calling and returning from machines is important
here too. As this currently stands this is not actually needed, but once
heredocs are in play we will have two possible places to return to at
the end of an interpolation sequence, so the state return stack maintained
by Ragel will determine whether to return to string mode or heredoc mode.
