[9.x] Attribute Cast Performance Improvements

[serpentblade]

Performance improvements to casts-related functions:

1. Reduce number of repeated calls to getCasts() in methods
2. Replace strncmp with str_starts_with in cast type check
3. Add cache for getCastType() to eliminate repeated string comparisons for cast types.

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Background:
I process millions of rows via scout and was curious if there were any obvious bottlenecks in the process to help speed things along. I discovered through profiling that casts-related code represented a significant number of overall calls and time spent processing my models.

In particular calls to getCasts() were unusually high as well and the time spent in getCastType() was higher than any other function.

First step was to update any methods where getCasts() was called multiple times. getCastType() seemed to be the primary culprit for the high number of calls to getCasts().

When profiling again this helped reduce the calls to getCasts() but the time spent in getCastType() was still rather high. I found isImmutableCustomDateTimeCast() still was using strncmp comparisons when str_starts_with would do instead. This change is not only an improvement to readability while matching the code of the other similar functions, but there's also a small improvement in performance with the change as an added bonus.

I then noticed that getCastType() can be cached as the results of checking the cast types are deterministic and do not need to be repeated for the same input.

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Results:
With these changes I saw a 5-25% reduction in processing time depending on the complexity, number of casts, and number of attributes of a particular model. From my tests I have noticed significant improvements on both eloquent load times and serialization.

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Potential risks:
This introduces a new static member variable $castTypeCache to Model that could cause conflicts if a subclass already defines one by that name. An alternative for this that is to introduce $castTypeCache as a static variable to the getCastType() method only, which is often considered bad form but does not introduce any risk of naming collision and results in the same benefit.

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End user benefits:

General users will likely see negligible benefits as the changes are only really beneficial when processing a lot of models. At scale, any bulk loading and serializing of models will yield note-worthy performance improvements.

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Other things to note about casts:

There are further significant performance improvements in caching hasCast(), however that is a bit more complex as there's no easy to trust that $casts or the results of getCasts() does not change during the lifecycle of a model. If a future version of Laravel introduced immutability to $casts and an expectation of determinism for getCasts() the expensive and repeated calls to hasCast() could be cached and save a significant amount of time.

[taylorotwell]

Thanks!