If the provided Hash doesn't have a default proc, we know for
sure that we'll never call into user provided code, hence the
string we allocate to access the Hash can't possibly escape.

So we don't actually have to allocate it, we can use a fake_str,
AKA a stack allocated string.

```
compare-ruby: ruby 3.5.0dev (2025-02-10T13:47:44Z master 3fb455ad) +PRISM [arm64-darwin23]
built-ruby: ruby 3.5.0dev (2025-02-10T17:09:52Z opt-gsub-alloc ea5c28958f) +PRISM [arm64-darwin23]
warming up....

|                 |compare-ruby|built-ruby|
|:----------------|-----------:|---------:|
|escape           |      3.374k|    3.722k|
|                 |           -|     1.10x|
|escape_bin       |      5.469k|    6.587k|
|                 |           -|     1.20x|
|escape_utf8      |      3.465k|    3.734k|
|                 |           -|     1.08x|
|escape_utf8_bin  |      5.752k|    7.283k|
|                 |           -|     1.27x|
```