[koorchik]

Depends on #31, please merge it after #31. Until then this PR targets the branch of #31, so it shows only its own commit.

Summary

Validation is 1.2-1.4 times faster, list_of and or 1.8 times, building a validator 1.5-1.8 times. The behavior does not change.

Operations per second, the same core, before and after:

Scenario	PHP 8.5	PHP 7.2
Validate a form with 12 fields	133564 → 164736 (x1.23)	63612 → 78537 (x1.23)
Validate the form with errors	183118 → 259537 (x1.42)	105325 → 142583 (x1.35)
Build a validator and validate the form	37313 → 57583 (x1.54)	22999 → 37266 (x1.62)
Validate nested data (meta rules)	16089 → 22333 (x1.39)	12295 → 16620 (x1.35)
Build a validator with meta rules	36792 → 64799 (x1.76)	27959 → 54170 (x1.94)
list_of with 10 items	170100 → 309592 (x1.82)	134855 → 243877 (x1.81)
or	795777 → 1419614 (x1.78)	670635 → 1056665 (x1.58)
iso_date	782155 → 1979233 (x2.53)	677039 → 1721187 (x2.54)
one_of with 26 values	2059354 → 3303844 (x1.60)	1635113 → 2764467 (x1.69)
The LIVR test suite, building a validator for every case	939 → 1437 (x1.53)	668 → 1033 (x1.55)

Building a validator matters most in PHP, as it is usually done on every request.

Changes

• validate() keeps the output of a field in variables instead of looking it up in the result twice for every rule.
• Util::isAssocArray() uses array_is_list() on PHP 8.1+ and a loop on older versions instead of making two arrays.
• A new validator shares the default rules instead of copying them, and registerRules() skips the merge when meta rules register the same rules.
• Rule builders given as Class::method strings are called as closures, which are not resolved on every call.
• parseRule() takes the first key without reset(), which copies the array.
• New getFieldRule($fieldName) returns the rules of a field as one rule, like in the JavaScript implementation. list_of, or and aliases use it instead of calling validate() for every value. It is described in the README.
• iso_date uses checkdate() instead of DateTime. This also fixes PHP 7.2, where DateTime turns 2769-12-31 into 2770-01-00, so some real dates were rejected and some wrong ones accepted.
• one_of looks values up in a set instead of in_array().

Testing

• PHPUnit on PHP 7.2 and 8.5: 77 tests pass, including a new test for getFieldRule(). phpcs passes.
• The LIVR conformance harness: 68 of 70 cases pass, as with Update the LIVR test suite and fix the bugs it finds #31.
• 200,000 random cases (random rules, including meta rules, aliases and auto trim, with random data) give the same output and errors before and after, on PHP 7.2 and 8.5.
• iso_date gives the same results as before for every date from 0000 to 9999 (months 00-19, days 00-39) and for 300,000 changed strings on PHP 8.5. On PHP 7.2 only the dates above differ.

Found while testing, not changed here: on PHP 8, trim, to_lc and to_uc throw a TypeError when a list or an object has an array in it, for example ['a', ['b']].