…m output.

To make this clean, refactored old `LoanPath` enum into a
`LoanPath` struct with a `ty::t` and a newly-added `LoanPathVariant` enum.

This enabled me to get rid of the ugly and fragile `LoanPath::to_type`
method, and I can probably also get rid of other stuff that was
supporting it, maybe.

… val now.

This is to fix a problem where I could not reliably map attach the
type for each loan-path to the loan-path itself because the same
loan-path was ending up associated with two different types, because
the cmt's had diverged in their interpretation of the path.

… eq types.

This is accomplished by:

1. Add `MatchMode` enum to `expr_use_visitor`.

2. Computing the match mode for each pattern via a pre-pass, and then
   passing the mode along when visiting the pattern in
   expr_use_visitor.

3. Adding a `fn matched_pat` callback to expr_use_visitor, which is
   called on interior struct and enum nodes of the pattern (as opposed
   to `fn consume_pat`, which is only invoked for identifiers at the
   leaves of the pattern), and invoking it accordingly.

Of particular interest are the `cat_downcast` instances established
when matching enum variants.

Includes differentiation between assigned_fragments and
moved_fragments, support for all-but-one array fragments, and
instrumentation to print out the moved/assigned/unmmoved/parents for
each function, factored out into separate submodule.

The tests use new "//~| ERROR" follow syntax.

Includes a test for moves involving array elements.  It was easier
than i realized to get something naive off the ground here.