Value is already tagged. Value::Acb blocks are allocated for lambdas (so
we can fit the value tag into the three bits we have available), but the
current layout is rather wasteful for this purpose. the type bits can be
stored together with parts of pointers, which in the lambda case will be
the scope the lambda captures. the expr could also be used, but Env is a
gc-allocated item and thus guaranteed to be aligned properly for tagging
Change-Id: Ia685875387c7795bc4a00d73d1ce3cfea84e7297
eldritch horrors
8108e8a760