destructor checker (dropck).

Largely adapted from pcwalton's original branch, with following
notable modifications:

Use `regionck::type_must_outlive` to generate `SafeDestructor`
constraints.  (this plugged some soundness holes in the analysis).

Avoid exponential time blowup on compile-fail/huge-struct.rs by
keeping the breadcrumbs until end of traversal.

Avoid premature return from regionck::visit_expr.

Factored drop-checking code out into dropck module.

Added `SafeDestructor` to enum `SubregionOrigin` (for error reporting).

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Since this imposes restrictions on the lifetimes used in types with
destructors, this is a (wait for it)

[breaking-change]

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