Wands as a core predicate

[giltho]

Depends on #387, only two commits on

Similar to predicates-as-core-predicates but with wands.
Controversial choice, I parse GILLIAN_WANDS__lhs_name__INTO__rhs_name to the equivalent of Wand { lhs: "lhs_name"; rhs: "rhs_name" }.
It's not beautiful. I'm starting to think that a better approach is to have a unique core predicate: <wand>("lhs_name", "rhs_name", ... ; ...)

If so, we should do the same thing for user-defined predicates, which should be of the form <user_defined_pred>("pred_name", in1, in2; out1, out2). I'll leave it for later if we get everything working .

For the review, an important thing to understand is that in a wand lhs(lins; louts) -* rhs(rins; routs), the in-parameters are lins @ louts @ rins and the out-parameters are only routs