[ruby-core:118994] [Ruby master Feature#20707] Move `Time#xmlschema` into core

Issue #20707 has been reported by byroot (Jean Boussier).

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Feature #20707: Move `Time#xmlschema` into core
https://bugs.ruby-lang.org/issues/20707

* Author: byroot (Jean Boussier)
* Status: Open
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### Performance

Converting `Time` into `RFC3339 / ISO8601` representation is an significant hotspot for application that serialize data in JSON, XML or other formats.


Right now this feature is currently available through the `time` default gem, so it rely on `Time#strftime`.
While [`strftime` could certainly be optimized](https://github.com/ruby/ruby/pull/11508), ultimately it's a very generic API so can't make a lot of assumptions about the output.

Whereas `ISO8601` is a very strict format, with very few dynamic part, so it's much easier for dedicated code to be well optimized, as the final size of the string can be computed upfront etc.

[I experimented with this and was able to get a ~5x speedup with a fairly straightforward implementation](https://github.com/ruby/ruby/pull/11510).

```
compare-ruby: ruby 3.4.0dev (2024-08-29T13:11:40Z master 6b08a50a62) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-08-30T13:17:32Z native-xmlschema 34041ff71f) +YJIT [arm64-darwin23]
warming up......

|                        |compare-ruby|built-ruby|
|:-----------------------|-----------:|---------:|
|time._xmlschema         |      1.087M|    5.190M|
|                        |           -|     4.78x|
|utc_time._xmlschema     |      1.464M|    6.848M|
|                        |           -|     4.68x|
|time._xmlschema(6)      |    859.960k|    4.646M|
|                        |           -|     5.40x|
|utc_time._xmlschema(6)  |      1.080M|    5.917M|
|                        |           -|     5.48x|
|time._xmlschema(9)      |    893.909k|    4.668M|
|                        |           -|     5.22x|
|utc_time._xmlschema(9)  |      1.056M|    5.707M|
|                        |           -|     5.40x|
```

### Usability

Aside from the performance reason, `ISO8601` is so common that having this functionality built-in without needing to require anything would make sense to me.
`xmlschema` is the most common reason why I require `time`, and in many case having it core would allow not to load it at all.







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