Make typechecker compositional

[catamorphism]

r? @nikomatsakis The typechecker previously passed around a boolean return flag to
indicate whether it saw something with type | (that is, something
it knows at compile-time will definitely diverge) and also had some
manual checks for the ty_err pseudo-type that represents a previous
type error. This was because the typing rules implemented by the
typechecker didn't properly propagate | and ty_err. I fixed it.

This also required changing expected error messages in a few tests,
as now we're printing out fewer derived errors -- in fact, at this
point we should print out no derived errors, so report any that
you see (ones that include "[type error]") as bugs.

[nikomatsakis]

Tim---this basically looks great! I didn't read it through in utmost detail, more skimmed over. This seems prone to fast bitrot so I thought it better to give r+ now and we can refine later. One improvement I would like is if you would modify the comment in typeck/check/mod.rs (or create one on check_expr) that explains the invariants here. For example, is it an invariant that if the thing has type bottom, it will hvae ty_bot (and not some type which has bot embedded?) Similarly for error? I'm not sure that is the right invariant, actually, since it seems like more work to maintain than is worth the trouble, but that's debatable and in any case I'd like to know.

[catamorphism]

@nikomatsakis No, that's not the invariant -- I wanted it to be, but wasn't able to implement it because if I tried to change ty::mk_t_with_id to return ty_bot when given, say, ~[ty_bot], I got memory corruption. So instead, "has_bot" and "has_err" are just type flags and the predicates type_is_bot and type_is_error check them. Thus, observationally, the invariant that if a thing has type bot then its type will be ty_bot is true, assuming the caller always uses the predicates and doesn't match on the type structure directly. I'll add a comment to this effect.

[catamorphism]

Added a comment -- @nikomatsakis r? just to make sure I was clear.