There are now three kinds of vars: global, local and local pure. Global
and local pure are exactly like in BAP, local ones are defined by their
initial set op and have the scope of a single lifted instruction.
The set op now handles both global and local vars, let is now pure and
binds only inside its body. Vars have static types, global and local are
always mutable, local pure naturally immutable.
The var op specifies the kind of variable to take from, and the
individual variable sets are separate. This corresponds to BAP's
behavior where the kind of variable is part of the identifier.
Variable content storage has also been rewritten and RzILBag removed.
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