Define primop, AST class, and HIR instruction class for the unbound slot value.

[robert-strandh]

Currently, the unbound slot value is defined as a Common Lisp
constant. But defining it that way has two negative consequences.
First, it can escape from the environment, and that is not
desirable. Second, during bootstrapping, it needs to be defined
specially with a different value, so the production version can not be
used.

By defining it as a primop, AST class and HIR instruction class,
during bootstrapping we can use the production version of code that
needs to check for the unbound value. Then, the AST evaluator and the
HIR evaluator can be configured to use the special bootstrapping
value, whereas the HIR-to-MIR translator will use the native value.