Prevent needless ASDF:MISSING-COMPONENT errors.

These were caused by a stale ASDF cache. ASDF's cache exists for the
scope of a dynamic binding. By changing our handling of
ASDF:MISSING-DEPENDENCY from a HANDLER-BIND to a HANDLER-CASE we cause
the handler to escape that scope.

Resolves #103
Merge pull request #104 from m-n/asdf-cache

Prevent needless ASDF:MISSING-COMPONENT errors.