Don't needlessly drop closures (issue #2603)

I hereby declare that messages sent from the same source arrive in order (Issue #2605)

Removing FIXME, owned is the correct type here. (Issue #2704)

Remove outdated FIXME (Issue #2703)

Updating test for spawning native functions (Issue #2602)

Removing bogus FIXME (Issue #2599)

The first benchmark shows about twice the throughput of the old system.

it seems that, to be truly useful, pure fns really need the
ability to modify their parameters.  alternatively, we could
rewrite the functions that modify their arguments to take/return.

trans now can safely assert that it never sees a type param

this also repairs the unsoundness in typing of unpack_slice,
which was silently converting a const ptr to an imm one.