Use more sophisticated ID generation for internal handles

[zombiezen]

Right now, there's a number of Go objects (i.e. user-defined functions and auxiliary data) that get "passed" through the SQLite library via a uintptr ID in a process-wide table. The way I generate these IDs is by simply incrementing an integer. For 64-bit systems, the probability of ID wraparound issues is exceedingly low, but it might be possible for high-volume applications on 32-bit systems to have collisions.

My observation is that the number of handles present at one time for any of these objects is relatively low (likely worst case low 100s), so I'm thinking a simple bitset would work well.