Fix: #788

`multi_json` rely on it, even though it was never documented as
public API.

Bringing it back as a method so it can emit a deprecation warning.

Bring back JSON::PRETTY_STATE_PROTOTYPE with a deprecation

Fix: #790

If we end up calling something that spills the state
on the heap, the pointer we received is outdated and
may be out of sync.

Stop caching the generator state pointer

We can't directly call `RBASIC_CLASS` as the return value of
`to_s` may be an immediate.

Handle non-string keys returning immediate values via `to_s`

See the pull request for the long development history: #743

```
== Encoding activitypub.json (52595 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     2.913k i/100ms
Calculating -------------------------------------
               after     29.377k (± 2.0%) i/s   (34.04 μs/i) -    148.563k in   5.059169s

Comparison:
              before:    23314.1 i/s
               after:    29377.3 i/s - 1.26x  faster


== Encoding citm_catalog.json (500298 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   152.000 i/100ms
Calculating -------------------------------------
               after      1.569k (± 0.8%) i/s  (637.49 μs/i) -      7.904k in   5.039001s

Comparison:
              before:     1485.6 i/s
               after:     1568.7 i/s - 1.06x  faster


== Encoding twitter.json (466906 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after   309.000 i/100ms
Calculating -------------------------------------
               after      3.115k (± 3.1%) i/s  (321.01 μs/i) -     15.759k in   5.063776s

Comparison:
              before:     2508.3 i/s
               after:     3115.2 i/s - 1.24x  faster
```

`c < 32 || c == 34` is equivalent to `c ^ 2 < 33`.

Found in: https://lemire.me/blog/2025/04/13/detect-control-characters-quotes-and-backslashes-efficiently-using-swar/

The gain seem mostly present on micro-benchmark, and even there aren't
very consistent, but it's never slower.

```
== Encoding long string (124001 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     5.295k i/100ms
Calculating -------------------------------------
               after     55.796k (± 3.4%) i/s   (17.92 μs/i) -    280.635k in   5.035690s

Comparison:
              before:    49840.7 i/s
               after:    55795.8 i/s - 1.12x  faster
```

`c < 32 || c == 34` is equivalent to `c ^ 2 < 33`.

Found in: https://lemire.me/blog/2025/04/13/detect-control-characters-quotes-and-backslashes-efficiently-using-swar/

The gain seem mostly present on micro-benchmark, and even there aren't
very consistent, but it's never slower.

```
== Encoding long string (124001 bytes)
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after     5.295k i/100ms
Calculating -------------------------------------
               after     55.796k (± 3.4%) i/s   (17.92 μs/i) -    280.635k in   5.035690s

Comparison:
              before:    49840.7 i/s
               after:    55795.8 i/s - 1.12x  faster
```

We should test compilation with `-msse2` because we need to
test with whatever arguments Ruby will be compiled with.

Fix i686 builds

Remove unnecessary conditional in escape_UTF8_char_basic.