Start at the first element instead of the address of the vector.

Un-XFAIL expr-alt.rs.

I don't don't totally understand the implications of this but it makes the
behavior consistent for all live edges, which is going to make joining the
arms of an alt expression work correctly.

This will make implementing default alt arms simpler.

Remove typestate_check reference to nonexistent ast.init node; rustc compiling itself notices what rustboot did not :)

I added a new field to the ast "ann" type for typestate information.
Currently, the field contains a record of a precondition bit vector and
postcondition vector, but I tried to structure things so as to make
it easy to change the representation of the typestate annotation type.
I also had to add annotations to some syntactic forms that didn't have
them before (fail, ret, be...), with all the boilerplate changes
that that would imply.

The main call to the typestate_check entry point is commented out and
the actual pre-postcondition algorithm only has a few cases
implemented, though the overall AST traversal is there. The rest of
the typestate algorithm isn't implemented yet.

This allows blocks to be used in conditional constructs where the block may
not ever execute: the drop glue will notice that it was never used and ignore
it.

Also, beef up the comments.