stop initializing ref_count in exchange_alloc

this is never read anymore
managed: rm RC_EXCHANGE_UNIQUE constant

this is no longer used, exchange allocations do not set ref_count
stop copying the tydesc in unique box take glue

the only user of the tydesc is ~fn, and it doesn't use this glue code
add a closure_exchange_malloc lang item

this makes the exchange allocation header completely unused, and leaves
it uninitialized