The extra delegation to `LazyAttributeHash` has non-negligible overhead.

Avoiding that delegation makes attributes access about 45% faster for
readonly (non-mutation) usage.

https://gist.github.com/kamipo/4002c96a02859d8fe6503e26d7be4ad8

Before:

```
IPS
Warming up --------------------------------------
    attribute access     1.000  i/100ms
Calculating -------------------------------------
    attribute access      3.444  (± 0.0%) i/s -     18.000  in   5.259030s
MEMORY
Calculating -------------------------------------
    attribute access    38.902M memsize (     0.000  retained)
                       350.044k objects (     0.000  retained)
                        15.000  strings (     0.000  retained)
```

After (with `immutable_strings_by_default = true`):

```
IPS
Warming up --------------------------------------
    attribute access     1.000  i/100ms
Calculating -------------------------------------
    attribute access      5.066  (±19.7%) i/s -     25.000  in   5.024650s
MEMORY
Calculating -------------------------------------
    attribute access    27.382M memsize (     0.000  retained)
                       160.044k objects (     0.000  retained)
                        15.000  strings (     0.000  retained)
```