Add a "closure information" field after each code pointer in
a closure. This field generalizes the "arity" field used by
the native-code compiler, in that it has room both for an arity
(always 0 in bytecode) and for the distance from the closure
to the first environment variable in the closure block.
This makes closures "self-described" and easy to scan for pointers:
everything up to the first env var is out-of-heap code pointers or
integers; everything after the first env var is a well-formed value.
At this point a bootstrap is needed.
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