Further work on typestate_check

[catamorphism]

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.