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.

The test was meant to verify that the typechecker correctly
allows a _|_ value (break, here) to be used in any context.
However, the compiler fails with an LLVM assertion failure.
I xfailed it, but wanted it to be on the record anyway.

The typechecker had a number of special cases for unifying types
with _|_ (as with checking if and alt). But, a value of type _|_
should be usable in any context, as such a value always diverges,
and will never be used by its immediate context. Changed unify
accordingly, removed special cases.

Use it to provide better error messages in the event of
glob-importing the same name multiple times.

and make sure that circular glob imports don't diverge.

Part one of adding globbing to resolve.rs: build up glob links. (Part two will follow those links if non-glob resolution fails.)

This lets us print better messages in situations with name
shadowing.

(Previously, an llvm assert was tripped.)