adjust plai's gc2 language to use roots as the arguments
to gc:cons and gc:closure instead of passing locs

This enables the important change, namely that get-root-set
no longer returns roots corresponding to the arguments
of the allocation function that we're in the middle of.

This means that a common error students have (forgetting
to chase the 'hd' and 'tl' pointers in their GC) is
harder to make now, since get-root-set never contains
those locations as roots. (In the past you would have had
to write some pretty non-obvious mutator program to get
that behavior.)