This makes it possible to pass the location of that space through to
nested blocks, resulting in less copying, taking, and dropping.

This makes the compiler slightly faster and 19k smaller.

A FIXME is to use 'move' semantics when returning the values from the
block -- don't bump the refcount and drop it again for the returning
block, but simply assign ownership to the receiver. To do this, we'll
need a way to (safely) scrub things from a block's cleanup list.

Now use RUST_LOG=rt::mem instead of RUST_LOG=rt.mem

A non-returning call should have a postcondition in which all predicates
are true -- not just a poststate. Otherwise, alt expressions where
one or more branches terminate in a non-returning call and others
initialize a variable get rejected.

Includes a test case.

* Non-returning calls should set all predicates to be true, not
  just the "this function returns" predicate

* Fixed a bug in the expr_alt case in tstate.states that wasn't updating
  the changed flag properly, then fixed *another* bug that was updating
  it too enthusiastically, but was masked by the first bug.

Changed the typechecker to correctly typecheck the declared variable
type in a for or for-each loop against the vector element type (for
a for loop) or the iterator type (for a for-each loop). Added a
test case.

Working on 2 argument spawn. The arguments seem to be copied correctly once, but then they get lost.

New tasks run, but their arguments get lost and something ends up in an infinite loop when the task exits.

This is no longer necessary since error logging is not affected by RUST_LOG.

Previously, block_ty returned the type of the terminating
expression of the block (or nil if said expression was absent).
I changed check_expr to write the type of that expression into
the annotation for the block itself, so now block_ty can use the
block's annotation.