perf(engine): create the parser and the name resolver on the first parse

[lisachenko]

What and why

Part of goaop/framework#730.

src/bootstrap.php is a composer files autoload entry, so it runs ReflectionEngine::init(new ComposerLocator()) on every request of every application that has this package installed. init() built the PHP-Parser parser, the emulative lexer and the name-resolving traverser right away. Most requests never parse anything; in goaop/framework, for example, parsing only happens when the transformed-code cache misses.

Change

• init() only stores the locator and the optional grammar version. It also resets any previously created parser and traverser, so a re-init() with another PhpVersion still takes effect.
• getParser() creates the parser on first use. A new private getTraverser() does the same for the traverser. parseFile() uses both.
• ComposerLocator stays eager on purpose. It looks up composer's own ClassLoader in spl_autoload_functions(), and goaop/framework later wraps that loader, after which a lazy lookup would find no ClassLoader.
• $parser and $traverser become nullable (?Parser = null, ?NodeTraverser = null) so that init() can reset them.

Measured. goaop/framework app, PHP 8.5, opcache with validate_timestamps=0, JIT off, built-in server, median of 200 warm requests. require vendor/autoload.php takes 229–242 µs → 87–91 µs, about 145 µs saved per request.

Backport. The 4.x branch, which serves PHP 8.4 installs, has the same eager init(). A cherry-pick of this commit applies to it.

Tests

• New tests/ReflectionEngineTest.php:
  • init() creates no parser and no traverser;
  • the parser is created once, on first use;
  • the PhpVersion given to init() is honoured: an 8.4 grammar rejects the pipe operator;
  • parseFile() creates both and resolves names.
• Full suite: 13827 tests OK, with the same 136 skipped and 2 incomplete as master. vendor/bin/phpstan analyse src: no errors.

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