Key optimizations:
- Add getKind() helper for O(1) type checking, replacing repeated
  4-condition pattern (val !== null && typeof val === "object" && ...)
- Add LeaKind type union for type-safe kind checking
- Lazy initialize Environment.reactivesBySource to avoid Map allocation
  overhead for non-reactive code
- Refactor stringify/coerceToString/getLeaType to use switch statements
  for faster dispatch
- Optimize asFunction to use getKind and for-loop over forEach
- Update builtins (fst, snd, variance, stdDev) to use getKind
- Replace spread operators with slice() for array copy (concat, reverse,
  shuffle, sort)
- Combine loops in variance/stdDev for better performance
- Add benchmark suite for performance testing (npm run benchmark)