Migrate most of Lucy core to _IMP.

Change implementing funcs to utilize the new-style _IMP function at the
site of the definition.  However, retain the old aliases where they are
invoked so that this large commit can remain very superficial.
Migrate LucyX core to _IMP.
Migrate the Lucy core's unit tests to _IMP.
Migrate the Perl bindings for Lucy to _IMP.
Migrate the C bindings for Lucy to _IMP.
Remove underutilized OVERRIDDEN macro.
Address direct calls within Lucy core.

Where the functions which implement methods in the Lucy core are invoked
directly rather than via dynamic dispatch, either rename them to _IMP,
change them to dynamic dispatch, or otherwise adapt.