feat(generator): faster lockless rand, debug port for pprof

[lizthegrey]

Which problem is this PR solving?

• Profiling data shows that we're spending way too long in rand.IntN() calls and in slice append() grow calls.

Short description of the changes

• Enable profiling.
• Replace the rand library with something that is lockless.
• We know how big the slice capacity must be at start, so pre-allocate the slice.