[m-rev.] for review: string switches using tries

[Michael Day]

Hi Zoltan,

> I don't have access to any real workloads in which switches on strings
> are a nontrivial part of the task, so I am missing real data for part (b).
> When other developers have had a chance to try out this diff and have found
> that it works for them, I was planning to ask for volunteer performance
> evaluators on the mercury-users list. I think it likely that Michael Day
> would be interested.

We have a few semidet switches on strings, eg. for CSS property names, 
which have lots of common prefixes like border-left, border-right, etc.

Peter Wang might like to test this, as he builds new compiler versions 
more frequently than I do :)

Michael

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