Optimization for htmlspecialchars function

[ArtUkrainskiy]

Optimization for htmlspecialchars function.

A dedicated php_htmlspecialchars function instead of the “universal” php_escape_html_entities_ex.
We work with ASCII-compatible encodings, we can employ byte-by-byte scanning and a lookup table to identify special characters. For c < 0x80, the lookup table is used; for potentially multi-byte characters, we continue to rely on get_next_char.
This approach provides a noticeable performance improvement for ASCII strings and some improvement for multi-byte strings due to more optimized logic.

Important!
We have changed the entity validation logic so that the maximum length of an entity name can no longer exceed LONGEST_ENTITY_LENGTH.

Benchmarks 4k char strings:
htmlspecialchars with UTF-8 encoding

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 |                      Test |  htmlspecialchars avg(ns) | htmlspecialchars_new avg(ns) |        diff(%) |
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 | 300 specialchars          |                     40657 |                        11559 |        251.73% |
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 | 300 spchrs. with entities |                     40986 |                        11550 |        254.86% |
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 | Clean ASCII string        |                     38348 |                         7401 |        418.15% |
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 | Cyrillic                  |                     50003 |                        38302 |         30.55% |
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 | Chinese                   |                     55860 |                        46308 |         20.63% |
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 | Japanese                  |                     56832 |                        46991 |         20.94% |
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htmlspecialchars with lang-specific encoding. Without encoding hint

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|                      Test  |  htmlspecialchars avg(ns) | htmlspecialchars_new avg(ns) |    diff(%) |
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| Cyrillic CP1251            |                     41518 |                        35148 |     18.12% |
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| Chinese  Big5              |                     57282 |                        47005 |     21.86% |
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| Japanese SJIS              |                     54703 |                        49887 |      9.65% |
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htmlspecialchars with lang-specific encoding. With encoding hint

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|                      Test  |  htmlspecialchars avg(ns) | htmlspecialchars_new avg(ns) |    diff(%) |
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| Cyrillic CP1251            |                     37185 |                        29197 |     27.36% |
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| Chinese  Big5              |                     48877 |                        38191 |     27.98% |
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| Japanese SJIS              |                     46282 |                        38317 |     20.79% |
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We may need more benchmarks.
This is not the final optimization, as get_next_char remains suboptimal for this function due to extra character-detection steps that aren’t required under the default flags.