Stop sqlite calling into moved Ruby objects

[djmb]

When you register a scalar function, aggregator, collation, trace handler or authorizer, we hand sqlite a pointer to a Ruby object. The garbage collector can move that object without updating sqlite's copy of the address.

The database's C struct now points at the collections holding those callbacks, so the mark function can pin them.

Trace and authorizer passed the database and read the callback out of an instance variable. They now pass the struct instead, which lives in malloc'd memory the collector never relocates. The busy handler already worked this way.

Writes to the new fields go through RB_OBJ_WRITE so the collector keeps them alive.

There is a test for each of the five that forces every movable object to move. None of them hold the object under test in a local variable, which would prevent that.

[djmb]

@flavorjones - I found this issue because I was getting random errors and crashes from a Rails app that used custom functions running parallel tests against SQLite.

I got Claude + Codex to then track down the other instances of unpinned objects that the GC could move.