[m-dev.] Trail registers

[Fergus Henderson]

Thomas Charles CONWAY, you wrote:
> 
> Is there any reason that the trail pointer is not treated like a
> normal virtual machine register?

Not particularly -- the only thing is that the trail pointer and ticket
counter are only used `#if MR_USE_TRAILING'.

It seemed nicer to keep them in mercury_trail.h, so that all the trailing
stuff is together, but with the re-entrant version, it probably makes more
sense for them to go in regorder.h.

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