Lots of work on typestate_check, seems to get a lot of the way
through checking the standard library.
* Added for, for_each, assign_op, bind, cast, put, check, break,
and cont. (I'm not sure break and cont are actually handled correctly.)
* Fixed side-effect bug in seq_preconds so that unioning the
preconditions of a sequence of statements or expressions
is handled correctly.
* Pass poststate correctly through a stmt_decl.
* Handle expr_ret and expr_fail properly (after execution of a ret
or fail, everything is true -- this is needed to handle ifs and alts
where one branch is a ret or fail)
* Fixed bug in set_prestate_ann where a thing that needed to be
mutated wasn't getting passed as an alias
* Fixed bug in how expr_alt was treated (zero is not the identity
for intersect, who knew, right?)
* Update logging to reflect log_err vs. log
* Fixed find_locals so as to return all local decls and exclude
function arguments.
* Make union_postconds work on an empty vector (needed to handle
empty blocks correctly)
* Added _vec.cat_options, which takes a list of option[T] to a list
of T, ignoring any Nones
* Added two test cases.
