cache class name

before:

```
// hessian1 encode: simple object  x   101,458 ops/sec ±9.30% (67 runs sampled)
// hessian2 encode: simple object  x   132,938 ops/sec ±3.23% (89 runs sampled)
// hessian1 encode: complex object x    90,243 ops/sec ±2.08% (93 runs sampled)
// hessian2 encode: complex object x    80,702 ops/sec ±5.94% (86 runs sampled)
```

after:

```
// hessian1 encode: simple object  x   132,363 ops/sec ±5.80% (83 runs sampled)
// hessian2 encode: simple object  x   138,711 ops/sec ±3.52% (89 runs sampled)
// hessian1 encode: complex object x    94,401 ops/sec ±1.15% (90 runs sampled)
// hessian2 encode: complex object x    90,484 ops/sec ±1.33% (97 runs sampled)
```