With embedded strings we often have some space left in the slot, which
we can use to store the string Hash code.

It's probably only worth it for string literals, as they are the ones
likely to be used as hash keys.

We chose to store the Hash code right after the string terminator as to
make it easy/fast to compute, and not require one more union in RString.

```
compare-ruby: ruby 3.4.0dev (2024-04-22T06:32:21Z main f77618c1) [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-04-22T10:13:03Z interned-string-ha.. 8a1a32331b) [arm64-darwin23]
last_commit=Precompute embedded string literals hash code

|            |compare-ruby|built-ruby|
|:-----------|-----------:|---------:|
|symbol      |     39.275M|   39.753M|
|            |           -|     1.01x|
|dyn_symbol  |     37.348M|   37.704M|
|            |           -|     1.01x|
|small_lit   |     29.514M|   33.948M|
|            |           -|     1.15x|
|frozen_lit  |     27.180M|   33.056M|
|            |           -|     1.22x|
|iseq_lit    |     27.391M|   32.242M|
|            |           -|     1.18x|
```

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>