Refactored working with built-in strings, symbols and small integers.

Refactored working with built-in strings, symbols and small integers.

- Implemented atom IDs for strings, symbols and small numbers, enabling
  equality checks via ID comparison
- Optimized string operations for faster property lookups and comparisons
- Removed short string inlining from njs_value_t structure

Performance improvements (arewefastyet/benchmarks/v8-v7 benchmark):
- Richards: +57% (631 → 989)
- Crypto: +7% (1445 → 1551)
- RayTrace: +37% (562 → 772)
- NavierStokes: +20% (2062 → 2465)
- Overall score: +29% (1014 → 1307)

In collaboration with Dmitry Volyntsev.