Draft: Fix #243: weak pointer to static array

Add function clear-static-vector-mark to clear the static vector mark for all the static vectors in lisp::*static-vectors*. This is added to *before-gc-hooks* so that the marks are cleared before GC so that GC can set it if the static vector is still in use.

Modify finalize-static-vectors to delete any element in lisp::*static-vectors* which has broken weak pointer. That means GC has freed the static vectors so we don't need to keep track of it anymore.

Finally, modify scan_weak_pointers to free the static vectors. The algorithm (from !188 (comment 13604)) is:

Here is an algorithm:

1. Traverse the weak pointer list and collect all of the weak pointers that point to unmarked static vectors. For lack of a more specific term we will call this the clearable list. (You can combine this traversal with the regular weak pointer clearing to save a traversal of the weak pointer list.)

2. Traverse the clearable list. For each weak pointer, if the value is an unmarked static vector, mark it. Otherwise, if the value is marked static vector, you know it shares a referent with another weak pointer so break the weak pointer and remove it from the clearable list. (If you don't you'll get a double free in the next step.)

3. Finally, traverse the clearable list a final time. The static vectors can be safely freed and all of the weak references on the clearable list can be cleared.

Step 1 established the invariant that all of the weak pointers on the clearable list point to an unmarked static vector.

Step 2 established the invariant that each weak pointer that remains on the clearable list references a unique dead static vector.