Gather constraints in program order

Gather constraints in program order

Provoked by a suggestion of Simon's, this patch makes a half-hearted attempt
to gather constraints in program order, so that we tend to report an error
at its first occurrence, rather than its last.  Examples:
	mdofail001, tcfail015

It's "half-hearted" because generally-speaking the typechecker does not
guaranteed to keep constraints in order; it treats them as a set.  Nevertheless
this very small change seems to improve matters, so it seems a good one.