[ASDAlexander77]

-mm=own: a class family spans one class hierarchy

Under -mm=own, any program that made both an Array<string> and an Array<int> (or
Array<number>) failed to compile, with errors inside DefaultLib's Array<T>:

lib.generics.ts: error: 'this value' borrows the result of '___Array.find<!ts.string>' and cannot be stored, returned or captured
note: 'Array<!ts.string>.find' could return a borrow or keep a parameter, but an override in its class family disagrees

OwnershipSignaturePass gives a virtual call the facts every member of its family agrees on. A
family was every function at that vtable position under that method name, in any class
(collectClassVTables). Two specializations of one generic share every slot and name, so
Array<string>.find (returns a borrow) and Array<int>.find (returns a number) were one
family, disagreed, and both lost their facts - though neither class extends the other and no
call through one can reach the other.

Families are now keyed by the root of the class's chain of bases as well (familyRoot: a base
is the first field of a class's storage, as extendsClass reads it). familyOf looks a call up
by its callee's class's root. A slot one of whose members has a root not known here (storage not
seen, or a chain deeper than 64) stays a single family, as every slot was before. The class
storages are now collected before the vtables.

Overrides within one hierarchy are unchanged: own_err_borrowed_result_family (an override that
returns a new object where its base returns a field) still errors.

Tests

• own/own_generic_specializations.ts: Box<string>.find and Box<int>.find returning a
  helper's result. Fails under own without the fix (string-only passes); runs under own, rc, gc
  and none, plus own's no-counting check.
• ctest Release: 6091/6091 passed.
• DefaultLib tests.ps1: gc, rc, none, own all pass in all four passes. new Array<string>("a", "b")
  next to new Array<number>(10) compiles and runs under own.

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