Optimize File.join common use case

`File.join` is a hotspot for common libraries such as Zeitwerk
and Bootsnap. It has a fairly flexible signature, but 99% of
the time it's called with just two (or a small number of) UTF-8 strings.

If we optimistically optimize for that use case we can cut down a large
number of type and encoding checks, significantly speeding up the method.

The one remaining expensive check we could try to optimize is `str_null_check`.
Given it's common to use the same base string for joining, we could memoize it.
Also we could precompute it for literal strings.

```
compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71eaf) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-18T12:10:38Z spedup-file-join 069bab58d4) +PRISM [arm64-darwin25]
warming up....

|              |compare-ruby|built-ruby|
|:-------------|-----------:|---------:|
|two_strings   |      2.475M|    9.444M|
|              |           -|     3.82x|
|many_strings  |    551.975k|    2.346M|
|              |           -|     4.25x|
|array         |    514.946k|  522.034k|
|              |           -|     1.01x|
|mixed         |    621.236k|  633.189k|
|              |           -|     1.02x|
```

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