Fix extra slice growth in rebuildOrderHeap

[jbowes]

rebuildOrderHeap was allocing a slice with a len of the max required size, then
appending unbound vars to it. This resulted in a slice with nbVars entries of
'0' to start with, followed by the actual unbound vars. Adding those unbound
vars would cause slice growth.

Ultimately, all but the last leading 0 value would be ignored by the queue,
and maybe that shouldn't have been there, as that var might already have been
bound?

Replace the slice alloc len form with the cap form, so there's no leading
'0's, and append doesn't cause slice growth.